Bassman AB165 with a screwed up Torres mod

[cjlectronics]

This amp was delivered to me with low volume. I was told it worked great until a Torres mod was performed on it and now the volume is low.

What I do not know is what does the Torres mod do to the amp? The attached photo highlights two caps that I was told were replaced. I have highlighted a few resistors because these values are way off from what the AB165 schematic said they should be. I understand that resistors may not be exactly what is on the schematic, but I highlighted the ones I thought didn't jive or even come close. INcidently, the photo is a snap shot of the circuit at the 12AT7.

The voltage value at pin 1 of the 12AT7 is 450v, the schemo lists it as 225v? The voltage at pins 3 and 8 of the 12AT7 is like 18v. The schemo lists it as 102v, but in the photo the value of the resistor that is missing is 470k, the schemo lists it as 470... for the Torres mod, is it supposed to be 470k??? I was about to replace this resistor with a 470 but I thought I should ask if the Torres mod changes this resistor to 470k?

Link to Bassman Ab165 schematic

[loudthud]

That's the resistor that goes to pins 3 and 8 of the 12AT7? 470K will never work there. No wonder the plate voltage is 450. Stick a 470 ohm in that place.

[Tom Phillips]

One good approach would be to restore the parts to the stock values. Then evaluate the performance of the amp and determine if anything else is wrong with the amp. Fix any problems before doing modes. I do not know what this specific Torres mod is but they are often extreme and IMO usually do not improve the performance.

[Chuck H]

+1
Restore the stock values. Then change it if you want to. Most Torres mods work fine (when done correctly). This seems to be an attempt to change the PI circuit values to AB763 (or "blackface") values. There is no real advantage to it without other circuit changes though.

The reason for the high voltage on pin 1 is that the tube isn't drawing as much current as it should be. When the tube draws current it will bring down the voltage. Changing that (missing) resistor from 470k to 470 ohms and connecting that 1M resistor (that is hanging now) will probably get the amp working fine.

That's some ugly work in there.

[Jazz P Bass]

Here is a link to a site where the gentleman rips the AB165 a new pie hole.
Some good thoughts on the whys & whereofs.
Link:AA864 Bassman vs AB165 Bassman

[52 Bill]

The photo shows a large power resistor by the output tubes, which sez to me that this was not an AB165 circuit. The mod works looks to be a bit shaky, I'd clean it up and either bring it back to stock or do the mod correctly based on the original instructions.

[Tom Phillips]

The photo shows a large power resistor by the output tubes, which sez to me that this was not an AB165 circuit. The mod works looks to be a bit shaky, I'd clean it up and either bring it back to stock or do the mod correctly based on the original instructions.

Good eye Bill. Based on that detail and the fact that it looks like original factory work I'd say the subject Bassman is an AC568

Tom

[cjlectronics]

Thanks for everyone's input. I do plan on putting the 470 ohm resistor in place of the 470k and then putting the .022 caps in and removing the .047 caps. Then put everything back to stock once I get an OK from the owner.

I'm glad you brought that up Bill, those power resistors are 150 ohm and connect at pin 8 to GND on each 6L6. So I assume these are for current limiting?? There is one on each power tube and both are 150 ohm. The amp label does have AB165 on it.

I wouldn't be surprised if this is someone's experiment project gone bad and I'm the lucky soul that is asked to fix it.

I am trying to find out a little more about this amp and have a call into the owner. This repair is on hold until I get more info tomorrow.

[Jazz P Bass]

The amp label does have AB165 on it.

If history is a guide, please do not take the label as gospel.
They have been known to be in error.

[Tom Phillips]

QUOTE=Jazz P Bass;236256]

The amp label does have AB165 on it.

If history is a guide, please do not take the label as gospel.
They have been known to be in error.[/QUOTE]
+1 on that. It is not uncommon to have incorrectly labeled tube charts.
I believe that the AC568 was a short lived model. The power amp circuit utilizes an unusual combination of fixed and cathode bias and is not known as one of the "good" circuits. It is a good candidate for conversion. Reference the posted schematics on Schematic Heaven to see the various circuit differences.

[Jag]

+1, I think that's an AC568.

I've had three AC568's and ALL of them said AB165 on the chart. They WERE AB165 when they left my bench

I've had several AA165's that said AA864 on the chart.

I've also seen a few Fender amps, from the '65-'68 period, that had variations or were mixes and had no matching "official" schematic.

[cjlectronics]

I replaced the 470k resistor with a 470 ohm and replaced the 5.6k resistor with a 22k. The low volume problem has been fixed and the amp is loud. I spoke to the owner and I was instructed to stop there. So, I will stop there and I have made it known that I will not warrant any work done. If it breaks again, I will fix it at an additional charge!

Regarding the schematic variations, the 22k and 0.1 cap to ground at the 12AT7 circuit has a 1 Meg resistor between ground and these two components. I looked at the AC568 circuit and that resistor is in that schematic but it is a 100 ohm. A big difference in value.

This reminds me of when I worked for a large company that made natural gas meters. They would release custom software to individual customers and do a half ass job of documenting what was done. Then when the customer called in for tech support, no one knew what the hell was going on. Scary!!

[hasserl]

I have one of those AC568 amps too. I've done some work on the preamp (Bass channel to 5F6A style), but left the cathode resistors in place. IMO this amp sounds terrific. There is nothing inherently wrong with combined cathode & fixed bias, I believe Dr Z uses that technique on one of his amps (don't recall which one right now). The cathode resistors provide for some natural compression from the bias shift that occurs as the current across the tube increases, which I kind of like.

[Chuck H]

There is nothing inherently wrong with combined cathode & fixed bias, I believe Dr Z uses that technique on one of his amps (don't recall which one right now). The cathode resistors provide for some natural compression from the bias shift that occurs as the current across the tube increases, which I kind of like.

I always wondered why designers and techs here adhere to the common mojo driven dislike for combination bias. I use both methods seperately on different amps. It really depends on what you want the amp to do. If I wanted a controlled amount of cathode bias sag I wouldn't hesitate to use a combination bias. One circuit I use in my el84 amps places a zener across the cathode resistor to limit the bias shift. Doesn't behave exactly the same as using both a cathode resistor and a -V grid supply, but it IS effectively a combination bias since the amps ARE cathode biased and then the bias is FIXED when it rises to the zener voltage.

[Tom Phillips]

...It really depends on what you want the amp to do...

Good point. There are individuals who are perfectly happy with the sound of their stock silverface amp model. They don't want a conversion or mod. They just want to keep it running like it did the day it left the factory.