guild head and poweramp question

mellowgerman

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I was just curious as to how i would run my guild head's preamp with a separate power amp. it has a preamp out jack and a power amp in jack...
what would my setup be (as far as guild head, power amp, and cab)? how do i connect them? is this altogether just a bad idea?
 

capnjuan

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mellowgerman said:
I was just curious as to how i would run my guild head's preamp with a separate power amp. it has a preamp out jack and a power amp in jack ... what would my setup be (as far as guild head, power amp, and cab)? how do i connect them? is this altogether just a bad idea?
Hi MG: together, the out/in jacks are an 'effects loop'. For the rest of it ... you have most of it figured out already. You need a shorter instrument jumper cable with 1/4" plugs - one end in the preamp out of your Guild head and the other end into the power amp in of some other amp to which whatever speakers you are using are connected. Not harmful to the power out section of your Guild head. CJ
 

mellowgerman

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very cool. thanks. i was just worried not having a load on the guild's power section would maybe hurt it
 

capnjuan

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mellowgerman said:
very cool. thanks. i was just worried not having a load on the guild's power section would maybe hurt it
Hi MG: no, your head's power tubes would just sit there idling at their bias current; on - but with nothing happening. Together, their internal impedances are the load on the primary side of the output transformer. In the 'whether it's worth the trouble department', the 4-tube preamp Guild heads have a 7247 / 12DW7 as the output driver and they operate at nearly 300 volts on the plates; that's compared to a relative modest 190-220 volts on more typical phase inverter/driver plates.

Those amps also have master volume controls. Depending on where the signal for the preamp out is taken; ahead, or downstream, of the master volume and driver, you could get 'mud in' at the power amp in jack of the second amp; that is, you could overload it but not necessarily damage that section of the other amp.
 

mellowgerman

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BTW i forgot to mention it's a Thunderstar Bass head... i don't know if that makes a difference or not.
controls are volume, bass, treble, and a tone switch.
 

capnjuan

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Ok; no Master volume but the remarks on the 7247 still apply - Guild used it all of its post-'66 designs. If you have a 12AX7 there, consider switching it out for a 7247 which is essentially 1/2 of a 12AX7 mated to 1/2 of a 12AU7 which has better plate voltage handling/heat dissipation characteristics; it's a worker bee ... put your tone bees in the 1st gain stage ... the 12AX7 you take out, your amp, and maybe your tone will thank you for it. CJ
 

matsickma

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capn and mellow,

Everything discussed above may be correct but my memory is indicating a potential problem.

In the era of the Thunderstar Bass amp Guild had promoted an amp concept for the Thunderstar Bass head that said they can be stacked together for increased power. Basically the Preamp OUT and Power amp IN were not planned to be used as an effects loop. Rather they were planned to be used in Master /Slave configuration. In that configuration the PreAmp Out would be connected to a 2nd Thunderstar Bass amp /speaker cabinet. If I remember correctly the Preamp Out of the Thunderstar Bass amp is a Splitter and not a re-route. In this configuration the preamp always supplies a signal to the power amp and Power section finals are driven. So you might damage the transformer if a load (resistor or speaker) is not connected to the Master Thunderstar bass amp. I have not been able to find a schematic for the Thunderstar bass amp. Does anyone have one so this can be checked?

M
 

capnjuan

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Thank you Mike; I'll take a look - see what I can see. CJ
 

capnjuan

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GM and Mike: Setting the late 60s Quantum and larger heads aside for the moment - not to mention that GM doesn't have one of those, the 2- and 3-tube preamp ThunderBass heads had Out/In jacks in slightly different locations in the circuits. Neither had a Master Volume or 2nd channel while the 4-tube, 6L6 and 8417 models were two-channel, had Master Volume, but no loops or anything looking like a master/slave functionality. These look like Effects Loop jacks; if no signal at the grid of the output tubes, they just sit there idling at their bias current. CJ

thunderbassloops.jpg
 

matsickma

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Ok capn,

That makes sense but I remembered having different results when I messed around with the Thunderstar Bass (TSB) amp.

I went to the basement and did a little experiment with my TSB (6L6 version).

1) I plugged a guitar into the TSB #1 Input.
2) I plugged a jack from the TSB Preamp OUT into the Input #1 of a Thunderbird (2nd generation).
3) I plugged a speaker cab into the Main out of the TSB.

What happened:

1) With the Thunderbird amp Off or in STBY. The amplified sound came out of the speaker cab plugged into the Main on the TSB amp.
2) The Vol control of the STB worked in the normal way and could adjust volume.
3) The Thunderbird amp ON with STBY ON and TSB ON with STBY ON: Standard sound came out of TSB cab and a Highgain version came out of the Thunderbird albeit a bit noisy.
4) Same as 3) but with the TSB in STBY mode no sound came out of the speaker cab (as expected) BUT the Highgain sound continued to come out of the Thunderbird amp. Gain could be adjusted by the TSB and Volume by the Thunderbird.

So based on that experiment it appears that any signal that enters the TSB from the normal input will route directly to the power amp stage of the TSB. If the Preamp OUT is used the signal is routed to both the Power section AND the Preamp. However if the STBY switch is disabled (i.e., it is off -no light on) then the signal passing through the TSB preamp is isolated from the Poweramp section. I didn't get to mess around with the Power amp section but the next experiment is to see if I can use the Power amp section can be used with the STBY switch disabled as a isolated Block amplifier. If so the TSB amp could theretically be used to Preamplify one signal and Power amplify a different signal.


Capn, I have to leave now and will be gone for a day. Maybe you can look through the TSB schematic to see if you can verify the routing I just described. Also, how can I get a copy of the TSB schematics?

M
 

capnjuan

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matsickma said:
... Also, how can I get a copy of the TSB schematics?
M
Send me your email address in a PM; I'll send you everything I have. CJ
 

capnjuan

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Hi Mike; I think your results depend on whether the standby switches in the ThunderStar Bass and the Thunderbird are ahead, or downstream, of the out/in jacks and whether the jacks are shorting type. GM's question was could he export his signal without wrecking anything and ... to a lesser extent, was it worth screwing around with. At least for the first part, I think the answer is yes. I was further guessing that he'd get no signal out of whatever speaker was tied to amp #1; this would be the case if the standby switch were downstream of the out/in jacks and it were in the Standby position.

But if he did, it still wouldn't harm his gear. If he got remote and local signal, then each output section would only see half of the signal strength. Note the diagrams above; on the left, standby is ahead of the jacks; on the right, it's downstream. I don't have a schematic for the ThunderStar and am not sure which Thunderbass GM has. I get the feeling these amps were sold with instructions. Regards, CJ
 

mellowgerman

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sorry i've been absent for a bit guys! i've been so excited about the starfire that i kinda forgot about the poweramp idea. but i'm back now.
thanks for all the help so far. regretfully though, i'm having trouble following you guys as i have limited knowledge of amps.
I have the TSB schematics if anyone needs them...
I'll definitely hold off on experimenting with the amp until i hear from you guys. thanks again!
 

capnjuan

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Hi MG; you can export your preamp's signal to any amp that has a jack labeled 'power amp in'. In the remote amp and depending on where the standby switch is in the circuit [up or downstream from the jack] and what type of jacks they are [shorting or non-shorting], you may or may not overload the output section of the remote amp; that is, get crappy sound. If you do and provided you don't dime it, you will not harm either your head or the remote amp.
 

mellowgerman

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hmm what exactly do you mean by "dime it"?
also, should the standby light be on or off when hooked up to the remote amp?
 

capnjuan

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mellowgerman said:
hmm what exactly do you mean by "dime it"? also, should the standby light be on or off when hooked up to the remote amp?
Hi MG; 'dime it' means turn the volume all the way up to ten. As far as the standby switches are concerned - yours and the remote amp's, you'll just have to experiment. So long as you keep the volume down, you can't screw anything up.

Below left: Guild 2-tube pre-amp Thunderbass shown. In normal operation, the signal follows the dark blue line and it appears on the output jack but doesn't go anywhere. It loops up the input jack, through the contact arm, and back down. If a remote signal were jacked in, the plug would open the contact arm breaking the normal circuit and the amp-in signal would follow the light blue line. The red dot is the junction of the standby switch (in green circle) and the line signal. If the standby switch is in the On Standby position, the output of the preamp is grounded out but the power section is still running and it can process a remote signal. So long as the standby switch is On Standby, this amp cannot have it's ouput section overloaded with both local and remote signals. If the standby switch is in the 'On' position, this circuit relies on the Amp-In jack to break the normal ciruit.

thunderloopse-1.jpg


In the amp on the right, a Guild 3-tube preamp Thunderbass head. The standby switch (green circle) is downstream from the Out/In jacks. In normal operation, the signal follows the dark blue line; up the 'amp in' jack, across the contact arm, and reaches the input (grid) of V2B. At that point, if there is a load connected at the Out jack, the signal flows through V2B and appears on the output jack. Since nothing is normally connected there, the signal bypasses V2B and continues in the circuit reaching the red dot; junction of the line and standby switch. If the standby switch is in the On Standby position, it will ground out everything; the normal local signal or a jacked-in remote signal. The contact arm on the 'amp in' jack disconnects the local signal and the remote signal bypasses V2B. When the standby switch is downstream of the Out/In jacks, the standby switch must be in the 'On' position if it's going to pump out a signal; its own or remote signal.

As indicated, the On Standby/On positions of the standby switch to export/import signals is governed by the type of jacks and the location of the standby switch in the circuit and as far as matsickma's experiments are concerned, we are overlooking the manufacturer's Yin and Yang here. There's a normal tendency to assume that what's on the schematic matches what's in the amp and that whatever's there is wired correctly and isn't burfed up.

At the risk of grinding my Gibson axe to make this point; in the pic below, the amp on top is a late 50s/early 60s Gibson GA20T (I own one like it). The amp on the bottom is a late 50s/early 60s Gibson GA20T ( I own the amp on the bottom of the pic ... dang you Mad Dog :evil: :lol: ). You'll note that in the amp on top, the output and rectifier tubes are on the left and the circuit board is on the right. On the bottom, the output and rectifier tubes are on the right and the circuit board is on the left ... and you get extra credit if you noticed that the circuit board on the bottom is populated by a lot of lumpy things - capacitors - while you can only see two capacitors on the circuit board on the top (the 'loop' is the footswitch cord) ....... the capacitors are on the bottom side. Finally, the preamp tubes in the upper amp match the schematic, the ones in the lower amp do not ... although the amp works.

leftright.jpg


My point is that just because a schematic says something, it doesn't necessarily mean that that's how the amp is built or that, however it's built, it works correctly. Watch the volume and don't wet the power cord before putting it in your ear. Best, CJ
 

matsickma

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Hey capn,

I have owned around 5 different Thunderbass amps over the years and none had the Preamp OUT/Power Amp IN feature. I remember reading that the Guild Thunderbass 1A model began to have the feature of stacking amps so would have the Pre Out/PowereIN option.

This would have occurred sometime after the Thunderstar Bass amp came out which is circa 1968.

Visually amps that are Thunderbass 1A will have the following features: 1) handle on top of the head, 2) designed to sit on top of the cabinet instead of suspended with the aluminum "bracket" and 3) have small rectangular vent holes on top rear of the amp head to vent the tube heat.

When you talk about 2- or 3 preamp Thunderbass heads are they 6L6 vs 8417 models or are they Thunderbass vs Thunderbass Quantums? (Note: At this time I assuming the Quantums are the earl models The Thunderbass Quantums with the 6550 were the last version of the Quantum model that probable were release sometime in 1969.)

Just curious.

M
 

capnjuan

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matsickma said:
I have owned around 5 different Thunderbass amps over the years and none had the Preamp OUT/Power Amp IN feature.
Dunno Mike; you must not have owned one of the models that did; like the ones in the schematics and MG's for example or, you owned one or more before they were present or after they were eliminated.

The 2- and 3-tube preamp models are single channel with Out/In jacks. By contrast, the 4-tube models - whether 8417 like mine was or 6L6 like some of yours - were two-channel (for guitar or bass each with their own circuit board and 12AX7), had a Master volume switch (the third 12AX7), and a 7247 driver/phase inverter - the 4th preamp tube. My 8417 model didn't have Out/In either. These are the title blocks from these two models 2-tube preamp, left; 3-tube preamp right:

titleblocks.jpg


Neither of these are Quantums; per schematics, they are both 6L6. My impression is that the early Quantums are identified by the 6GF7 screen voltage regulator - 'the tube in the middle' like John K's - and were either 8417 like his (and mine was) or 6L6 and the later Quantums also had a 6GF7 but were fitted with 6550s instead.
 

matsickma

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Hi capn,

I see you were talking about Thunderstar bass amps. I thought you were using Thunderbass amps as examples.

Never mind. I screwed up.

Also I am pretty sure my Thunderstar bass amp is one of the later modes like your schematiic from 1969. It is one of the "blue paint" models.

M
 
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