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Originally posted on the old forum. Slightly edited here...great info from both John & matsickma!
Date Posted: 04/10/2005 4:42 PM
Posted By: john_kidder
OK Folks, I finally have to admit that I know nothing at all about nothing, and throw myself on the mercy of my elders and betters.
I bought a Thunderbass head a while ago, just had it refurbished and cleaned up, and now I have it here in my music room. And I plug it in, and plug in my old Guild (still with PAFs, haven't yet swapped in the Franz's), and it sounds just wonderful and warm and fat and all those things I had hoped for. But I've been an acoustic guy lo these many years, with a brief stint into computers and music with Roland in the early '80s. And I'm confused by the simple controls on this old amp.
So here we go:
there are separate inputs for Channels 1 & 2, on two separate sets of input jacks marked "Bass" and "Normal". Does this mean that there are two separate amplifier channels? And that one of them is biased or otherwise set up to be more suitable for the bass register, with the other for regular guitar input?
if two instruments are plugged into either the "Bass" and "Normal" sides, then I presume that the Volume and Tone controls on that side of the amp control the output of both instruments. Conversely, if one instrument is plugged into the Bass side and one into the Normal side, then the Volume and Tone controls for each side control that instrument (apologies for such basic questions, but I'm really totally ignorant here)?.
Am I right in assuming that the Volume and Tone controls for each section are controlling a pre-amp within each, and the Master Volume controlling the main power amp?
If there is no instrument plugged into one of the channels, I presume one should keep the volume on that channel right down to prevent noise to the power amp?
The Master Volume obviously governs both channels. This now gives me, the acoustic player, three ways to control the volume from the guitar. If I want to have control without having to reach for the amp all the time, I presume I don't want to run the guitar wide-open in the normal course of things. Is it standard practice then to run the Volume full on for each (pre-amp?) channel, and then control output with the Master (power amp?)Volume, or to open up the Master Volume and control the individual channel Volumes?
If I open up the Treble control on the amp, I seem to get a much brighter sound than was available even with full treble on my Roland Jazz Chorus 55. If I were still playing with synthesizers, I'd say it had a much sharper "attack". Twangy, not jazz-y. Is this a treble boost, rather that just a cut-off filter? And how does this interact with the Tone controls on the guitar - I presume those are high cut-off filters - if I damp the high end out at the guitar, does the Treble control on the amp then boost it again? It can't create the missing frequencies cut off at the guitar, so what's the net effect?
There are two slide switches beside each Volume input: on the Bass side, with two positions; on the Normal side, with three. These seem to act like simple high cut filters - how do they interact with the Treble and Bass tone controls?
What's the Polarity switch for? Just to compensate for old-style ungrounded power plugs being plugged in "upside down"? Or does it have something to do with polarization of the output of the instruments themselves?
On the back of the amp, I have a "Hum Balance" control. Balance "hum"? Who? what? Between channels?
What's the difference between the "Accessory" output and the "Speaker" output?
That's all for now. Thanks in advance to anyone who is able to deal with a poor neophyte, and help me out with some or all of this.
BTW, Jay, if you happen to be reading this, be aware that it's in part because of your great write-up that I jumped into this at all. And I have to say that I feel a difference in the sound, as well as hearing it. Lovely. Just lovely. Once I learn how to control it, I'm sure I'll be happy as a clam.
Thanks all.
Cheers,
John Kidder
--------------------------------------------------------------------------------
Date Posted: 05/11/2005 9:47 PM
Posted By: matsickma
Hi John,
From your description you seem to have a pretty good understanding of the amp operation. I currently have two of these amps from two different time periods - about 4 years apart. I have owned others and noticed subtle differenced in style. A few I'll describe. The Thunderbass Amp may very well be one of the longest running models in the Guild amp history. This amp had 5 or 6 different cosmetic changes over the period starting around 1964 and running to about 1971. Some were subtle others were a re-style.The origional amp had the beige tolex and a aluminum support bracket that hooked the head to the speaker cab. The support bracket doubled as a handle for the head. You basically flipped the head around and carried it by the support. The initial amps were packaged in a cabinet with little ventelation and were constructed from 1/2 inch thick particle board. the package wasn't great but continued to evolve. It didn't take long before Guild found the top of head cabinet tolex getting dirty and torn-up so they added 4 beige color rubber feet to the top of the head. You may notice many of the heads, even the later black ones, have 4 holes located in the cornner on top of the head. As time march on the electronics design stayed essentially the same but Guild changed the head cabnet style to black and used 3/4 " pine and plywood. Ventelation was added in the rear and the top. Additional handles were attached and wraps for power chord were included in the back panel. For the most part the knobs and front control panel stayed the same.
I will try to answer or comment on you items and follow your number system.
1. The amp does have two different preamp channels and as you suspect each are optimized toward Bass or Guitar. I pulled a schematic and looked at the input paths and found that different RC values for an input High Pass Filter (HPF) are called out for each channel. Also note that Input 1 has higher gain than Input 2 (i.e., no series resistor). This is true of both channels. Note: I can see the RC values are different but the poor quality of the schematic made it hard to read actual numbers.
2. Your description of item 2 is how I see it. However although both input 1 and 2 can be used simultaneously I suspect the main intent was to provide a best match to the input strength of the instrument pickup or signal. In this era keeping preamap distortion low was a perferred sound. As described above 1 and 2 had different gain (in actually one had more loss). The addition of the Bass and Normal channel also server the practical approach of flexibility of instrument applications and offering two people to use the same amp or to use the amp along with a microphone. (Note: If you look at the Guild catalog from around 1965 with the Hi-5 on the cover two guitarists are sharing a Guild Thunderbird guitar amp.
3. The Master volume effectivly controls the main power amp but it reall controls the maximum output of the preamp. The power amp section is biased on at all times. It is wide open. The input signal to the power amp adjusts the volume and that is achieved by controlling the output signal of the preamp.
4. Your description is the best method. The noise could also be kept acceptable is the tone know is below the 3 oclock position. Above that the amps get noisy. Guild did take steps to minimize unused channel input noise. Input 1 uses one jack type with the tension shorting straps. If a plug is not inserted the input jack is shorted. They didn't use this on input 2. Most likely because on the additional attenuation and resulting lower gain.
5. The Master volume controls the output of the preamp. It's initial application woul have been to control the TOTAL output or volume when both channels are used. It functions as a two channel mixer with master volume. In todays era it takes on an additional dimension. Since we guitarist usually want some distortion of the preamp and power amp you can tweek and adjsut the channel and master volume controls to get your "right" sound. All stages of the preamp offer differences in the tube drive. This amp is pretty simple by the multi gain stages of modern tube amps but can be adjusted to get the right blend of preamp tube tone and the power tube tone. I have used the two channels along with a ABY switch to get the right blend of sound or tone.
6. Guild amps in the 1960's had great treble tone. The amps offered substantially greated upper frequency response than the 1950's models. Pick up a Guild Thunder 1 or Thunder 1 RVT, swap in a better speaker, and you will hear Brightness at its best. I once read a article wher the Thunderbass amp was compared to a Fender Bassman. They were similar in design but the author noted Guild did not roll off the upper end like the Bassman. It has been a long time since I played a Bassman so I can't compare but I agree with you on the bright characteristic of the Thunderbass. In the schematic the treble and bass controlls look like LPF, HPF respectivly. (Note: I invite others to comment on these circuits.). You can rais and lower the tone of your instruments from the guitar controls or amp tone controls. Your can compensate or augment bass or treble. Since it is unlikely the filter parameters are identical between the guitar and amp your probably can't completely netrulize the effect. However you ear may think you off-set each other but a Spectrum Analyzer may show slight differences.
7. The 3 position tone switches act as you describe. The middle position is Flat, the other two adjust different LPF settings on the normal side. I forgot to look at the Bass side but this switch may have different HPF values. The filter values are all different between the switches and tone controls but they can be adjusted to offst or augment each other.
8. Polarity Switch is just as you described. I is to swap hot terminals on the 110 V AC 2 prong lines. This was a quick adjustment to reduce electrical shock and hum. Certaintly a dangerous arrangement for bands in those days. These old amps do pose a danger. I recommend you have the amp updated to a modern power cord and remove or disconnect the switch. At the least get a rubber mat to stand on when playing. However this won't keep you from getting shocked if you touch other electronic gear with the switch in the wrong position.
9. Hum balance is a Tube amp thing. I don't have the proper explaination at this time. It does effect the 60 Hz hum rejection of the tube amp finals. I need to get more info herebut have been seeing feature on most vintage tube amps. It may be related back to the two prong AC power. (Does anyone have a explaination?)
10. The "Speaker" output is recommended as the first connction to the speaker cabinet. The "Accessory", later stated as a"External speaker" in other amps is another connection that is another tap off of the output transformer. This is added to add to allow you to add an additional speaker cabinet or load while keeping the output tube impedance match in the proper range. If not compensated for the tubes will fail prematurely. (Transistor power amps can handle a wide range of of output impedances. The impact is a reduction of available power but not destruction of the amp. (Note: For either amp you don't want the output to see a short circuit. However Open circuit is not a problem for a transistor amp but can destroy the output transformer of a tube amp.)
Guild offered a few other Bass amps. The Thunderstar Bass amp is very similar to a Thunderbass amp with only a BASS channel. However this amp has a specially designed feature that allowed you to drive multiple units in a Master/ Slave arrangement. Theoretically you can keeping adding them together with a speaker cab to get any amount of volume you want. The Quantum Amp was the most powerful of the Guild bass amps. The early models look just like a Thunderbass amp. The discrimator is a small metal label stuck on the front of the Thunderbass control panel. This amp use different power tubes ran at a higher voltage. The updated Quantom model has a very large head cabinet. The amp is constructed with a preamp section and a seperate power amp section. This amp is about 100W. It was designed as a bass amp but in the normal channel, and driven hard, starts to sound like a Marshall. The smallest Guild bass amp was the "Baby Bass" which is a Thunder 1 guitar amp with 2-10 inch speakers!? The last of the Bass amps is the Maverick Bass amp with about 30 W of power and a 15" speaker.
I have a passion for Guild guitars and amps. I am glad to share my info with you. Your questions and observations were very interesting and fun to respond to.
Sincerly,
matsickma
--------------------------------------------------------------------------------
Date Posted: 05/12/2005 4:03 PM
Posted By: john_kidder
Thanks, matsickma. Very helpful indeed. I'll go back to playing with the amp with a whole new understanding of the basics, and much tighter control.
Can't thank you enough,
John
Date Posted: 04/10/2005 4:42 PM
Posted By: john_kidder
OK Folks, I finally have to admit that I know nothing at all about nothing, and throw myself on the mercy of my elders and betters.
I bought a Thunderbass head a while ago, just had it refurbished and cleaned up, and now I have it here in my music room. And I plug it in, and plug in my old Guild (still with PAFs, haven't yet swapped in the Franz's), and it sounds just wonderful and warm and fat and all those things I had hoped for. But I've been an acoustic guy lo these many years, with a brief stint into computers and music with Roland in the early '80s. And I'm confused by the simple controls on this old amp.
So here we go:
there are separate inputs for Channels 1 & 2, on two separate sets of input jacks marked "Bass" and "Normal". Does this mean that there are two separate amplifier channels? And that one of them is biased or otherwise set up to be more suitable for the bass register, with the other for regular guitar input?
if two instruments are plugged into either the "Bass" and "Normal" sides, then I presume that the Volume and Tone controls on that side of the amp control the output of both instruments. Conversely, if one instrument is plugged into the Bass side and one into the Normal side, then the Volume and Tone controls for each side control that instrument (apologies for such basic questions, but I'm really totally ignorant here)?.
Am I right in assuming that the Volume and Tone controls for each section are controlling a pre-amp within each, and the Master Volume controlling the main power amp?
If there is no instrument plugged into one of the channels, I presume one should keep the volume on that channel right down to prevent noise to the power amp?
The Master Volume obviously governs both channels. This now gives me, the acoustic player, three ways to control the volume from the guitar. If I want to have control without having to reach for the amp all the time, I presume I don't want to run the guitar wide-open in the normal course of things. Is it standard practice then to run the Volume full on for each (pre-amp?) channel, and then control output with the Master (power amp?)Volume, or to open up the Master Volume and control the individual channel Volumes?
If I open up the Treble control on the amp, I seem to get a much brighter sound than was available even with full treble on my Roland Jazz Chorus 55. If I were still playing with synthesizers, I'd say it had a much sharper "attack". Twangy, not jazz-y. Is this a treble boost, rather that just a cut-off filter? And how does this interact with the Tone controls on the guitar - I presume those are high cut-off filters - if I damp the high end out at the guitar, does the Treble control on the amp then boost it again? It can't create the missing frequencies cut off at the guitar, so what's the net effect?
There are two slide switches beside each Volume input: on the Bass side, with two positions; on the Normal side, with three. These seem to act like simple high cut filters - how do they interact with the Treble and Bass tone controls?
What's the Polarity switch for? Just to compensate for old-style ungrounded power plugs being plugged in "upside down"? Or does it have something to do with polarization of the output of the instruments themselves?
On the back of the amp, I have a "Hum Balance" control. Balance "hum"? Who? what? Between channels?
What's the difference between the "Accessory" output and the "Speaker" output?
That's all for now. Thanks in advance to anyone who is able to deal with a poor neophyte, and help me out with some or all of this.
BTW, Jay, if you happen to be reading this, be aware that it's in part because of your great write-up that I jumped into this at all. And I have to say that I feel a difference in the sound, as well as hearing it. Lovely. Just lovely. Once I learn how to control it, I'm sure I'll be happy as a clam.
Thanks all.
Cheers,
John Kidder
--------------------------------------------------------------------------------
Date Posted: 05/11/2005 9:47 PM
Posted By: matsickma
Hi John,
From your description you seem to have a pretty good understanding of the amp operation. I currently have two of these amps from two different time periods - about 4 years apart. I have owned others and noticed subtle differenced in style. A few I'll describe. The Thunderbass Amp may very well be one of the longest running models in the Guild amp history. This amp had 5 or 6 different cosmetic changes over the period starting around 1964 and running to about 1971. Some were subtle others were a re-style.The origional amp had the beige tolex and a aluminum support bracket that hooked the head to the speaker cab. The support bracket doubled as a handle for the head. You basically flipped the head around and carried it by the support. The initial amps were packaged in a cabinet with little ventelation and were constructed from 1/2 inch thick particle board. the package wasn't great but continued to evolve. It didn't take long before Guild found the top of head cabinet tolex getting dirty and torn-up so they added 4 beige color rubber feet to the top of the head. You may notice many of the heads, even the later black ones, have 4 holes located in the cornner on top of the head. As time march on the electronics design stayed essentially the same but Guild changed the head cabnet style to black and used 3/4 " pine and plywood. Ventelation was added in the rear and the top. Additional handles were attached and wraps for power chord were included in the back panel. For the most part the knobs and front control panel stayed the same.
I will try to answer or comment on you items and follow your number system.
1. The amp does have two different preamp channels and as you suspect each are optimized toward Bass or Guitar. I pulled a schematic and looked at the input paths and found that different RC values for an input High Pass Filter (HPF) are called out for each channel. Also note that Input 1 has higher gain than Input 2 (i.e., no series resistor). This is true of both channels. Note: I can see the RC values are different but the poor quality of the schematic made it hard to read actual numbers.
2. Your description of item 2 is how I see it. However although both input 1 and 2 can be used simultaneously I suspect the main intent was to provide a best match to the input strength of the instrument pickup or signal. In this era keeping preamap distortion low was a perferred sound. As described above 1 and 2 had different gain (in actually one had more loss). The addition of the Bass and Normal channel also server the practical approach of flexibility of instrument applications and offering two people to use the same amp or to use the amp along with a microphone. (Note: If you look at the Guild catalog from around 1965 with the Hi-5 on the cover two guitarists are sharing a Guild Thunderbird guitar amp.
3. The Master volume effectivly controls the main power amp but it reall controls the maximum output of the preamp. The power amp section is biased on at all times. It is wide open. The input signal to the power amp adjusts the volume and that is achieved by controlling the output signal of the preamp.
4. Your description is the best method. The noise could also be kept acceptable is the tone know is below the 3 oclock position. Above that the amps get noisy. Guild did take steps to minimize unused channel input noise. Input 1 uses one jack type with the tension shorting straps. If a plug is not inserted the input jack is shorted. They didn't use this on input 2. Most likely because on the additional attenuation and resulting lower gain.
5. The Master volume controls the output of the preamp. It's initial application woul have been to control the TOTAL output or volume when both channels are used. It functions as a two channel mixer with master volume. In todays era it takes on an additional dimension. Since we guitarist usually want some distortion of the preamp and power amp you can tweek and adjsut the channel and master volume controls to get your "right" sound. All stages of the preamp offer differences in the tube drive. This amp is pretty simple by the multi gain stages of modern tube amps but can be adjusted to get the right blend of preamp tube tone and the power tube tone. I have used the two channels along with a ABY switch to get the right blend of sound or tone.
6. Guild amps in the 1960's had great treble tone. The amps offered substantially greated upper frequency response than the 1950's models. Pick up a Guild Thunder 1 or Thunder 1 RVT, swap in a better speaker, and you will hear Brightness at its best. I once read a article wher the Thunderbass amp was compared to a Fender Bassman. They were similar in design but the author noted Guild did not roll off the upper end like the Bassman. It has been a long time since I played a Bassman so I can't compare but I agree with you on the bright characteristic of the Thunderbass. In the schematic the treble and bass controlls look like LPF, HPF respectivly. (Note: I invite others to comment on these circuits.). You can rais and lower the tone of your instruments from the guitar controls or amp tone controls. Your can compensate or augment bass or treble. Since it is unlikely the filter parameters are identical between the guitar and amp your probably can't completely netrulize the effect. However you ear may think you off-set each other but a Spectrum Analyzer may show slight differences.
7. The 3 position tone switches act as you describe. The middle position is Flat, the other two adjust different LPF settings on the normal side. I forgot to look at the Bass side but this switch may have different HPF values. The filter values are all different between the switches and tone controls but they can be adjusted to offst or augment each other.
8. Polarity Switch is just as you described. I is to swap hot terminals on the 110 V AC 2 prong lines. This was a quick adjustment to reduce electrical shock and hum. Certaintly a dangerous arrangement for bands in those days. These old amps do pose a danger. I recommend you have the amp updated to a modern power cord and remove or disconnect the switch. At the least get a rubber mat to stand on when playing. However this won't keep you from getting shocked if you touch other electronic gear with the switch in the wrong position.
9. Hum balance is a Tube amp thing. I don't have the proper explaination at this time. It does effect the 60 Hz hum rejection of the tube amp finals. I need to get more info herebut have been seeing feature on most vintage tube amps. It may be related back to the two prong AC power. (Does anyone have a explaination?)
10. The "Speaker" output is recommended as the first connction to the speaker cabinet. The "Accessory", later stated as a"External speaker" in other amps is another connection that is another tap off of the output transformer. This is added to add to allow you to add an additional speaker cabinet or load while keeping the output tube impedance match in the proper range. If not compensated for the tubes will fail prematurely. (Transistor power amps can handle a wide range of of output impedances. The impact is a reduction of available power but not destruction of the amp. (Note: For either amp you don't want the output to see a short circuit. However Open circuit is not a problem for a transistor amp but can destroy the output transformer of a tube amp.)
Guild offered a few other Bass amps. The Thunderstar Bass amp is very similar to a Thunderbass amp with only a BASS channel. However this amp has a specially designed feature that allowed you to drive multiple units in a Master/ Slave arrangement. Theoretically you can keeping adding them together with a speaker cab to get any amount of volume you want. The Quantum Amp was the most powerful of the Guild bass amps. The early models look just like a Thunderbass amp. The discrimator is a small metal label stuck on the front of the Thunderbass control panel. This amp use different power tubes ran at a higher voltage. The updated Quantom model has a very large head cabinet. The amp is constructed with a preamp section and a seperate power amp section. This amp is about 100W. It was designed as a bass amp but in the normal channel, and driven hard, starts to sound like a Marshall. The smallest Guild bass amp was the "Baby Bass" which is a Thunder 1 guitar amp with 2-10 inch speakers!? The last of the Bass amps is the Maverick Bass amp with about 30 W of power and a 15" speaker.
I have a passion for Guild guitars and amps. I am glad to share my info with you. Your questions and observations were very interesting and fun to respond to.
Sincerly,
matsickma
--------------------------------------------------------------------------------
Date Posted: 05/12/2005 4:03 PM
Posted By: john_kidder
Thanks, matsickma. Very helpful indeed. I'll go back to playing with the amp with a whole new understanding of the basics, and much tighter control.
Can't thank you enough,
John
