Audio Guild Tube amp

Hello

New to the forum
I am looking for info on Audio Guild Amps. I am having a weard reverb problem with mine and am looking for a schematic or advice. I get a static sound in the reverb section only. It sounds like the tv on Poltergist. It is faint when turned down and gets louder as i turn the reverb up. I get a faint reverb signal and get the same static with the reverb tube and tank removed. This amp is a 2 channel amp with reverb and trem . 2x7591 power tubes , 1x6GH8 trem and reverb , 2x12ax7 Phaze and preamp. I also replaced all of the electrolyics in the amp.

Any thoughts welcome
BD

Hi BD and welcome to LTG: what about the reverb pot? It could have crud trapped between the wiper and the surface of the resistive element. Does spinning the reverb control have any effect when you are plugged into channel 1? Checked for loose connections with a chopstick? The only other comment I have is that static is more likely to come from a bad resistor than a bad cap. Good luck with your project! CJ

Hi Capnjua

It does the same thing in both channels with or without a signal applied. I did clean the pots pretty good but i can try it again. There are 3 transistors in this amp and i now at least one is used in the reverb curcuit. Could one of those gone bad from age or someone else messing around in the amp. They look original and appear to be a little rusty . My power tubes are glowing blue but i had tested on my tube tester and they tested good but now the amp is crackling from time to time . Im going to grab a different set later when i get home and try them out. I was also thinking about converting the amp to 6l6's but this amp isnt cathode biased and i am unsure how to do it . Any thoughts

Thanks for the warm welcome
BD

Hi BD; have you had a chance to clean the tube sockets and the tube pins? CAIG / DeOxit directly into the socket, push the tube in and out several times to scrub off the oxidation. If you have brass-bristle brush, try scrubbing the pins with it; dirty sockets and pins can cause crackling and spitting.

If it were mine, I'd want it running well before considering messing with 6L6s; there are plenty of good NOS and new 7591s around; normally rated around 30 watts. There are several issues with making your amp into a 6L6 amp; as seen below and assuming the bias supply were adequate, the pinouts for 7591s and 6L6s are different.



The filaments in 6L6 draw a large amount of current; possibly more than your transformer can supply. Further, the 'glowing blue' is a function of the existing bias, not the condition of the tubes. If the amp isn't cathode-biased but doesn't include individual pots for bias adjustment, you either have cut in plate resistors to calculate voltage drop and then manipulate the bias resistor (keep changing it) until you get the desired idle current or cut in a bias pot and on and on and on ... going to 6L6s is a lot of work and won't fix what corrently ails the amp.

I'd say that if you can't stop the tubes from glowing blue, you won't have to worry and any static, the reverb or anything else. Did you change the bias cap during your re-cap? Did you replace the bias resistor that sets the bias? You might poke around on the Weber BB and ask if anyone is familiar with biasing a 7591 amp, that is, what it's bias settings should be. Sorry that I can't be any more helpful on this last point. CJ

Hi Capnjuan
So i should address the sockets and get the bias set befor woring about anything else. If i get the amp biased wright the tubes shouldnt glow is this correct. I have had tubes glow befor in amps but not both at the same time. I nopticed that oner of the little wires inside of the reverb tube is glowing red hot. Is this a bias problem also?
the amp is fixed bias but i am thinking about converting it to cathode bias. I will try cleaning the pots to get rid of the crackle

I didnt take filliment voltage into consideration when i thought about a 6L6 swap.

thanks
keith.

baddog said:

So i should address the sockets and get the bias set befor woring about anything else. If i get the amp biased wright the tubes shouldnt glow is this correct.

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Hi Keith; correct on both. Get the amp to baseline and then see if you like it enough to go to the extra trouble ... and it's a lot of work. That's correct; all you should see in the output tubes is the little orange glowing dot of the top of the heaters; nothing else.

baddog said:

I nopticed that oner of the little wires inside of the reverb tube is glowing red hot. Is this a bias problem also?

Click to expand...

Maybe but not the way you might think; if it's a 12A_7, like the output tubes, you should see a little orange spec. If it's red hot, the tube could be getting near the end of its useful life, or the tube's cathode resistor - that sets the tube's bias - has drifted out of tolerance, or - unfortunately, the plate resistor has drifted out of tolerance and doesn't block enough DC allowing the plate to get hot and the tube to idle too high.

baddog said:

the amp is fixed bias but i am thinking about converting it to cathode bias.

Click to expand...

Well, it makes for a simpler amp with some loss of power out but ... if you are staggering around in the dark playing blind man's bluff without a schematic, at some point it would be easier to do that than struggle with trying to maintain it as a fixed-bias amp. CJ

All of the resistors are carbon composit and i am sure have drifted over the years. What about buying new metal film resisters and replacing all of the resistors in the amp? Would that mess with the tone?

I think cathode bias is the way to go. I have a better understranding of that tybe of biasing. All i need to do is remove the bias voltage wire from the board and connect it to ground and install a 5 or 10 watt resistor and E-cap from cathode of both tubes to ground. Is this wright? If so what size resister do you recomend starting with? Ampeg used 5791a tubes and had a 140ohm 10 watt resistor and a 25uf 25v cap.

Thanks
BD

baddog said:

What about buying new metal film resisters and replacing all of the resistors in the amp? Would that mess with the tone? It would change the tone but it's pretty hard to make a value judgement; it might wind up tighter, drier, more accurate, clearer .. to many people, that would be good. If it used to be sloppy and gritty and you or someone else thought that was good, then you might think that all that work was a waste. If you wanted to straddle the problem, you might consider only changing the plate and cathode resistors; adds reliability but at a lesser sacrifice in basic character. All i need to do is remove the bias voltage wire from the board and connect it to ground .. It just needs to be disconnected from the circuit board and either safed off or clipped off; not connected to ground. and install a 5 or 10 watt resistor and E-cap from cathode of both tubes to ground. Well, the 25uf/25v cap is right but I don't know what would be the right value resistor ... that's a question I can't answer. You might search Schematic Heaven for cathode-biased 7591 designs and see what they use; otherwise, ???

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CJ

Hi

according to Mikes Tube amp page you have to disconect the the fixed bias source that goes to the 2 resisters that connect to the grid to ground . See Short cut

http://people.cornell.edu/pages/mt24/Am ... _bias.html

For the cathode resistor Mikes Tube Amp page reccomends starting with a 500 ohm and make adjustments from there.

Thoughts
BD

baddog said:

according to Mikes Tube amp page you have to disconect the the fixed bias source that goes to the 2 resisters that connect to the grid to ground . They and I are right; the wire from the bias supply to the two resistors mounted on the pc board is the wire to be cut or made safe. For the cathode resistor Mikes Tube Amp page reccomends starting with a 500 ohm and make adjustments from there. Thoughts None really; if the rest of their stuff is reliable, I don't see why not.

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Good luck! CJ

Make or buy a bias probe or do the trick where you desolder the ground wire from the tube cathode to chassis. Replace the wire with a 1 ohm, one watt resistor (1% tolerance) and you can directly check with a multimeter what the tube is dropping. Check at weber's for the correct amount of current for 7591's.
Considering that these amps aren't that common, I wouldn't start screwing with the circuit until I had the original design running as best it could. If I wanted some kind of cathode-biased 6l6 amp, there's plenty of kits for that.

My opinion, FWIW.

I did switch the amp to cathode bias and replaced all opf the biasing resistors in the amp. The crackling apeared to stop intil i tried a different set of tubes then it reapeared. I put the original tubes back in and it crackled for a secon then stoped. The origenal tubes fit in the sockets really tight but the old ones i tried where loose. I didnt get a chance to clean the sockets yet or acualy bias the amp but the amp did sound better with the changes i did. The 2 bias resistors for the 12ax7 tubes where so old that i couldnt read the ohm rings bur they did read 900ish ohms. I didnt have a 1 k resistor so i use a 1.5k fenderish value instead.
Any Thoughts
BD

baddog said:

... The 2 bias resistors for the 12ax7 tubes where so old that i couldnt read the ohm rings bur they did read 900ish ohms. I didnt have a 1 k resistor so i use a 1.5k fenderish value instead.

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Hi BD; provided that stage is operating at 180-200vdc, I don't think you'd notice too much. If, like some reverb recovery stages; Mesa, Gibson, others where the reverb 'return' side runs at as low as 115-135vdc, you might notice some loss of signal strength. CJ

I did notice this morning that the i do get a slight ticking sound with the static noise in the reverb section. I am asuming that the noise is coming from the vibrato section. I read that some old fender amps had a problem with the trem causing a ticking noise in the amp and they cured it with a .o2 cap from the photo cell to gound . If i remember corectly.

thanks
BD