• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Output transformer replacement... Worth it?

apehead

Member
2006-10-26 5:27 pm
Hello,

Yesterday I purchased a Peavey Butcher Amplifier. I like the sound, but don't love it. I think this amp really does have some potential though. The tubes (3 ECC83's and 4 6L6-GC's) are stock, and since this amp hasn't been manufactured since 1986, I have already ordered new ones (all JJ). It is also fixed bias for the output stage, and apparently Peavey was (is) notorious for setting the bias cold (12 mA through four push-pull 6L6's?). So I have also ordered parts for an adjustable bias mod. I was also considering upgrading the output transformer from stock. Does anyone know who manufactured Peavey Transformers? Will it make an appreciable difference in sound ($100+ worth)? What brands are reccomended? Should I wait until the other modifications are done before spending the dough? I haven't measured the impedance of the output primary yet. Also, what "brand" should I be using in DuncanAmps Tone Stack Calculator program? The tone circuit in the Butcher looks like a fender physically, but the amp was designed to compete with the Marshall JCM 800.

Here is a link of the schematic:

Peavey Butcher Schematic
 

m6tt

Member
2007-08-08 12:12 am
you'll probably get more change in sound from modifying tone stack, bias points and other more or less minor changes than by replacing the transformer. Another major way to change an amp's sound is by converting the phase inverter to a different style, although be aware you should not use one which will provide less gain unless you make it up elsewhere in the circuit. I would also try basic tube rolling, or even converting stages to use different tubes (by changing cathode resistors, anode resistors and possibly caps), like 12at7 for phase inverter and 12ay7 for input tube. RCA manual or other amp schematics which use these tubes will have appropriate values. Transformers do quite a bit, but more of the tone comes from the circuit itself.
 
I agree with M6tt that other mods will do more than replacing the OT. I think that would be a complete waste of time and money (changing the OT).
A bias mod will improve the sound a lot on these amps since like you have found out already they are very cold biased.
If this were my amp, I would convert it to a cathode bias output stage and get it more in the class-A region and drop the fixed bias all together. The amp will drop from 120watts to something like 80-90watts, still way more than you ever need. If you want to be advanced, you can install a switch and with it switch between fixed/cathode bias if you like.
 

m6tt

Member
2007-08-08 12:12 am
Sounds good semperfi! I entirely agree...I'd just add that since the cathode bias resistor is going to be dissipating a lot of heat that it should be rated appropriately, and I'd also recommend using separate cathode resistors for each 6L6, this will allow you to use tubes without the need for *identical* matching. Since it'll be in class A, you may want to consider adding a fan to the chassis if it isn't well ventilated. One reason they may have biased it cold in the first place might have been to avoid the excess heat.
 

apehead

Member
2006-10-26 5:27 pm
Thanks for the replies. I took a look at the tone stack on Duncan amps Tone Stack Calculator, and compared to the tone stack response of a silverface twin reverb. This tone control is pretty awful. I imagine that has a lot to do with the sub-par sound of the amp overal, and I'm sure the cool biasing only makes the amp sound even more lifeless. If I can find the proper pots (hard to find long leg PCB mounted audio taper with 7/16 bushings!) I am definitely "blackfacing" this Peavey as soon as possible.
 
butcher bias mod

Can someone go into a little more detail about the Butcher bias mod? I just picked one up and was curious about a couple things. First, will a linear or audio pot work better for this mod? Also does the wiper or the outside lug go to ground? I've also seen people adding a small pulldown resistor to one lug of the pot. Does the pulldown resistor go to ground or to R52?

Any advice would be appreciated.

Thanks.
 
xfmr marking

I am not trying to encourage transformer change, but am curious if there are any other markings other than the Peavey part number in the schematic.

If it's pre-1986, it might conform to the common marling system of an EIA code and date code.
xxxx-yyyy

xxxx being a 3-4 digit number and yyyy being a 4 digit number.

If there is, I'll take a stab & see if I recognize the number. There were an awful lot of small and large transformer companies who could make functional if not stellar items to a customer's spec...it seems to be a modern trend to reflect upon classic designs and who made what components, with some suppliers achieving a 'status' in the consumer's eyes based on different parameters than the amplifier manufacturer's expectations.

Sometimes the customer only gave requirements, and the supplier used their expertise to determine the design and manufacturing methods...so the customer may not have even specified winding technique...just impedances, frequency response and power requirement/temperature rise, etc.

Somewhere in a box I have a couple lists of EIA codes from the 70's-80's.

I contacted the EIA once to see if that info was still available, maybe a collective historical list, and I couldn't even reach anyone :clown: who had a clue what I was asking about.
 
Peavey was (is) notorious for setting the bias cold (12 mA through four push-pull 6L6's?).

That can't be right. 12 mA, either going through one tube or four, is going to sound horrible, if it even works at that current. This amp is fixed biased, meaning there is no cathode resistor. The 6L6s are happy anywhere between 30-60 mA EACH. There should be -45 to-50V at pin 5 of each 6L6. If it's higher, (more negative), the 220K resistors R30 and R31 need to be changed to a higher value.
 
It doesn't sound like it was much of a competitor for the JCM800... agree with the posts above, get some new tubes in it and bias up the output tubes so they get some heat. Then look at the preamp/phase splitter.

Still, what kind of tone are you looking for? Maybe the 6L6 output tube isn't what you want - maybe a swap to an EL34 or 6550 since you're modding the bias anyway. That would change up the sound some for sure. If you're sticking 6l6, I have the large bottle chinese KT66s in my 6L6 guitar amp now and I really like the sound of those - better than the Winged C 6L6...