Want to use preamp section ONLY on 100w tube amp

[Mesa]

First thread here, I'm excited to start learning a ton on tube amps here!

Here's my situation, I sold a buddy a 5150 212 combo, he cut the speaker cabinet off and turned it into a 50w 5150 head, then he took the output transformer out of his bugera 6260 (5150 clone) and shoved it into the 5150 combo (opened up the amp beyond belief!!) Well after buying an orange th100 my buddy just gave me the OT-less bugera 6260, part of me wants to make it into a 2xEL84 or 2x6V6 amp running with the old 5150 OT, and part of me wants to know what I'd have to do to turn it into a standalone preamp.

My questions, if I run the amp with no power tubes in it, will that overpower the 12ax7's in the preamp? if so, can I correct that without buying a different PT?
Do I still need to buy a small (tiny?) OT to use the preamp only?

long story-short, what do I need to do to run the Preamp standalone?

Thank you for your time! I look forward to learning more!

P.S. I don't want to buy an engl e530 etc, I want to do this for fun, not necessity, I have a 5150ii 100w head and a carvin legacy 100w head.
I really want to get my hands dirty with tube amps, I've bias modded a few amps, repaired a few amps, modded a PV valveking, did the OT swap with my buddy on that 5150 combo, now I want to truly understand the entire amp, so that I can build/mod for myself and friends!

[soundguruman]

If the power tubes are removed, there will be less load on the power supply, and the DC voltage will go up. Will it be "too" high? It will be pretty high.
By removing the power amp from the equation, the sound will be altered significantly. A lot of the characteristic sound comes from the power amp included...
You might want to reduce the B+ so that the preamp runs about the same voltage, as if the power amp were still in the circuit.
But still, the filament voltage cannot be reduced to the preamp tubes without shortening the life of the preamp tube. So the filament remains the same, while the B+ is compensated.
Is this a crazy idea? of course.

[big_teee]

First thread here, I'm excited to start learning a ton on tube amps here!

Here's my situation, I sold a buddy a 5150 212 combo, he cut the speaker cabinet off and turned it into a 50w 5150 head, then he took the output transformer out of his bugera 6260 (5150 clone) and shoved it into the 5150 combo (opened up the amp beyond belief!!) Well after buying an orange th100 my buddy just gave me the OT-less bugera 6260, part of me wants to make it into a 2xEL84 or 2x6V6 amp running with the old 5150 OT, and part of me wants to know what I'd have to do to turn it into a standalone preamp.

My questions, if I run the amp with no power tubes in it, will that overpower the 12ax7's in the preamp? if so, can I correct that without buying a different PT?
Do I still need to buy a small (tiny?) OT to use the preamp only?

long story-short, what do I need to do to run the Preamp standalone?

Thank you for your time! I look forward to learning more!

P.S. I don't want to buy an engl e530 etc, I want to do this for fun, not necessity, I have a 5150ii 100w head and a carvin legacy 100w head.
I really want to get my hands dirty with tube amps, I've bias modded a few amps, repaired a few amps, modded a PV valveking, did the OT swap with my buddy on that 5150 combo, now I want to truly understand the entire amp, so that I can build/mod for myself and friends!

If the Bugera was working before removing the OT, why did he butcher it?
You can by OT all day for medium bucks.
Here's one for $68 for 4 6L6s.
Fender Output Transformer, Showman, Dual Showman, Twin Reverb, Bassman 100, 4/8/16 Ohm
Probably a better transformer than the one he took out.
I would put it back together and sell it and do something else.
Be aware that there are voltages in the amp that can kill you!
Good Luck,
T

[J M Fahey]

If you want to, you can run that preamp alone.
Go and try it.
I would never try to build something different (2x6V6 thingie or whatever) but return it to normal working state.
The transformer suggested above is very good.
Your friend is crazy, what was he thinking about?

[Mesa]

I guess he considered the head a throwaway since its a bugera, which is kind of how i feel now that its mine for free with no OT.
So let me make something clear, I want to tinker with something, I want to know what voltages to look for with my meter and what means what etc
If i bought a $70 OT, I'd still need 4 6l6's or el34's, so $70 more, the heads worth 250-325 TOPS, so I'd make $110-$185 dollars.... what will that buy me? not much...
aren't OT's pretty much matched impedances for a certain amount of tubes, as well as rated for a certain amount of power, for example, an OT for an amp with 4 6l6's is going to work fine for an amp with 4 el34's, but wouldn't necessarily support 4 kt88's?

Therefore, I can run just the two inside tubes, and use the OT from the 5150 combo, it was made for 2 6l6 tubes but saturated waaayy to early for the "60" watts the amp was rated for
hence my wanting to throw 2 6v6 tubes in their place, or maybe even either solder in 2 el84 sockets, or buy 2 THD yellow jacket's.

For the record, my friends 50("60") watt 5150 head with the bugera OT sounds rediculous, its so full and lively, its so clear and punchy and makes my 5150ii sound like mud, he lacks a little bottom end due to the wattage difference, but its still ballsy, it's unlike any 5150/6505 I've ever heard.

[J M Fahey]

If you want to tinker, fine.
Don't think about resale value and as a money earning machine, people just want the new gadget and are not interested in used/older/repired stuff. Period.
If you find that Bugera will be useful to you repaired , go ahead.
if you want to do "something" but don't want to spend $$$, wire an output jack to the preamp out and used it as such, maybe driving an SS power amp.
You can get a rackmount or DJ type one for peanuts.
Building "something else" such as 2x6V6 or whatever is not practical *AT ALL* in such an amp, full of connectors and built on crowded double face PCBs.
Worst case, junk the full PCB and drop a Ceriatone or Hoffman or something PTP board there and just use the transformers and chassis to build something interesting, a Hiwatt or a Plexi or JCM800.
But modding what you have is just not practical.
And you still will need that transformer and power tubes.

[olddawg]

I don't get the Bugera OT being superior? I can't imagine anyone getting the idea to even swap it out. From what I understand the OT is a weak point in Bugera amps. Have you a/bed the amps on the same speaker setup? Maybe it's a saturation thing. I know that many legendary amps used the cheapest off the shelf parts available. Does the Bugera OT get hot? Is it the bias. It doesn't make sense. Any thoughts out there? I would gut what you have and build something using the chassis, cab, and transformers. Use the same tubes of your main amp if money is an issue. You always need spares. Yeah, you could always use it as a preamp with the output tubes pulled.

[Mesa]

If you want to tinker, fine.
Don't think about resale value and as a money earning machine, people just want the new gadget and are not interested in used/older/repired stuff. Period.

For the sake of my questions, pretend the amp is worth absolutely nothing, that's how I'm treating the situation

Building "something else" such as 2x6V6 or whatever is not practical *AT ALL* in such an amp, full of connectors and built on crowded double face PCBs.
Worst case, junk the full PCB and drop a Ceriatone or Hoffman or something PTP board there and just use the transformers and chassis to build something interesting, a Hiwatt or a Plexi or JCM800.
But modding what you have is just not practical.
And you still will need that transformer and power tubes.

This I don't get, arent 6v6's the same pin pattern? and people pull 2 6l6's out of the outside sockets of their 100w heads all the time, they just have to change the impedance selector to compensate the OT, but in my case, I'll be using an OT used for 2 tubes anyways.
I've heard of zener diodes to reduce the voltage to the 6v6's so they don't burn up, and the JJ 6v6's handle higher than normal voltages for a 6v6 anyways, still with a 14 watt plate dissipation rating
2 of those puppies for 25-28watts of somewhat lower volume distortion...

[Mesa]

I don't get the Bugera OT being superior? I can't imagine anyone getting the idea to even swap it out. From what I understand the OT is a weak point in Bugera amps. Have you a/bed the amps on the same speaker setup? Maybe it's a saturation thing.

It's completely a saturation thing, the bugera is a 100w rated OT slapped in a 2x6l6 5150 combo (50-"60"watts)
the stock 5150 combo OT is a joke, its beyond tiny and saturates at 3, beyond that the amp just sounds worse and worse, muddier and muddier, and would never hold its own in the mix vs a 100w head.
Now with the bugera OT the 5150 combo just plain won't saturate, it stays clear through 10 on the master volume, its pretty neat
Don't get me wrong, I'm sure the OT out of the 5150 100w head would have made the combo sound even better, but we didn't have 2 5150's that we didnt care about lying around at the time, we did however have a bugera 333xl and a 6260 lying around, the 333xl was rumored to have much to gain from a jsx/ultra+ OT, so we popped that OT out and slapped in into the 5150combo, then I fixed the broken 6260 and my friend offered me the 333xl to have, so i swapped the 6260 OT into the 333xl (same part numbers fyi) and took the 6260 instead, with the hopes of doing something fun


Now I feel crazy...

[big_teee]

It's completely a saturation thing, the bugera is a 100w rated OT slapped in a 2x6l6 5150 combo (50-"60"watts)
the stock 5150 combo OT is a joke, its beyond tiny and saturates at 3, beyond that the amp just sounds worse and worse, muddier and muddier, and would never hold its own in the mix vs a 100w head.
Now with the bugera OT the 5150 combo just plain won't saturate, it stays clear through 10 on the master volume, its pretty neat
Don't get me wrong, I'm sure the OT out of the 5150 100w head would have made the combo sound even better, but we didn't have 2 5150's that we didnt care about lying around at the time, we did however have a bugera 333xl and a 6260 lying around, the 333xl was rumored to have much to gain from a jsx/ultra+ OT, so we popped that OT out and slapped in into the 5150combo, then I fixed the broken 6260 and my friend offered me the 333xl to have, so i swapped the 6260 OT into the 333xl (same part numbers fyi) and took the 6260 instead, with the hopes of doing something fun


Now I feel crazy...

The primary inpedance would be lower on the 100w OT for 4x6L6.
It would have the 2.1k primary.
The Primary impedance for 2x6L6 would be 4.2k.
I would think this be difference you are hearing.
T

[Mesa]

The primary inpedance would be lower on the 100w OT for 4x6L6.
It would have the 2.1k primary.
The Primary impedance for 2x6L6 would be 4.2k.
I would think this be difference you are hearing.
T

Can we please stop focusing on the fact that the bugera OT is in the 5150....

[big_teee]

Can we please stop focusing on the fact that the bugera OT is in the 5150....

Sure!
Sounds like you know what you're going to do, go for it.
Good Luck,
B_T

[Enzo]

OK.

You want to use just the preamp? Go ahead. Don't change anything. So without the power section, B+ will rise from 350 to 400 at the preamp, or whatever. SO what? You are not asking us for it to be identical, are you? If it takes on a new character, there you go - something new. If you DO want it more or less as it was, then think about the power supply. The B+ node that rebounds the most is the first one, and the preamp never used that. So all the subsequent nodes are drops from resistors. All you need do is increase the size of a resistor. Zener tricks are needed to get the FIRST node down to size - that is not what you need.

The amp expects that draw from the 6L6 power stage to set voltages. Putting a tiny EL84 stage in its place won;t compensate, it would still underdraw and increase voltages. But as above, so what if voltages rise?

Easy as that.

The price for my advice is this: you gotta hear a little more about transformers. I am not trying to make you change your project, but in my opinion, you have mis-analyzed this, and if you base future work on faulty assumptions, you may not get where you want to be. And you DID talk about installing a new one of some sort.

I suspect the change in tone is from impedance differences (and perhaps even winding resistances) between the two transformers, NOT core saturation. Peavey transformers are pretty robust. People often hear the clean channel start to break up at about 4 or 5 on the little Classic 30 and think it is the power tubes. But it is the phase inverter. A different transformer can make the phase inverter work differently, sound differently. Remember your power amp includes a negative feedback loop with the transformer right in the middle of it. You might find experimenting with that NFB plus the differences in presence adn resonance circuits to be fruitful.

You mentioned the expense of new transformers. A hacked amp is not going to have much sale value, its value is in the completion of the project and the resulting satisfaction. Put your dollar value on that instead of on potential sale price. Just my opinion.

6V6 has the same socket wiring, yes, but not identical needs in support circuitry. So not a drop in solution. And I would probably want to reduce the first node voltage some for them.

EL84 are tiny and have dfifferent drive needs. To use them, you'd need to pull a drop off the power supply. I think that would be easier than 6L6s in a way. But you'd have to rework the phase inverter for it.

REally, you can wed any power amp to any preamp as long as signal levels coming out of one are close to the needs of the input of the other. It may not make sense to do so, but that isn;t the point.

[J M Fahey]

Now with the bugera OT the 5150 combo just plain won't saturate, it stays clear through 10 on the master volume, its pretty neat

This I don't get.
Just swapping an OT will do *nothing* to change the *monster* gain structure of a 5150.
No way.
The signal is already clipping hard by the 3rd gain stage ... and it has 5 gain stages, go figure.
Sorry.

[Mesa]

First of all, THANK YOU FOR YOUR TIME AND HELP!!!!
I mean it, I'm not trying to be arrogant, I completely understand that what I want to do is *slightly* over my head
That being said, I've been in and around a few amps and know how to conduct business in there safely, so I'm ready to learn

The price for my advice is this: you gotta hear a little more about transformers. I am not trying to make you change your project, but in my opinion, you have mis-analyzed this, and if you base future work on faulty assumptions, you may not get where you want to be. And you DID talk about installing a new one of some sort.

I suspect the change in tone is from impedance differences (and perhaps even winding resistances) between the two transformers, NOT core saturation. Peavey transformers are pretty robust. People often hear the clean channel start to break up at about 4 or 5 on the little Classic 30 and think it is the power tubes. But it is the phase inverter. A different transformer can make the phase inverter work differently, sound differently. Remember your power amp includes a negative feedback loop with the transformer right in the middle of it. You might find experimenting with that NFB plus the differences in presence adn resonance circuits to be fruitful.

I'm not against learning more about transformers, or NFB.
To be clear though, I was trying to say that the 5150 w/bugera OT was "more clear" not "more clean" and I'm refering to the lead channel when cranked with downtuned guitars playing through it (chugga chug) it stays clear and bright for lack of a better word(I don't want to imply that it was simply more trebly)
But I will keep reading on amps, OT's, PT's, NFB etc

You mentioned the expense of new transformers. A hacked amp is not going to have much sale value, its value is in the completion of the project and the resulting satisfaction. Put your dollar value on that instead of on potential sale price. Just my opinion.

I have no problem writing off the cost of the amp, and to a point I'm willing to shell out some cash for transformers, besides, worst case scenario they will come in handy in some other sort of project

6V6 has the same socket wiring, yes, but not identical needs in support circuitry. So not a drop in solution. And I would probably want to reduce the first node voltage some for them.

so zener diodes for 6v6's and increase in resistance of existing resistors for 12ax7's?

EL84 are tiny and have dfifferent drive needs. To use them, you'd need to pull a drop off the power supply. I think that would be easier than 6L6s in a way. But you'd have to rework the phase inverter for it.
.

That sounds much more difficult

Do you know the best literature to study all the things I'm trying to figure out? Books, good websites, sticky threads on forums etc...
Thank you once again for your time!