Reuse PT and OT for very different build?

[Erik Aa]

Hello. I have a PT and an OT from a gutted 100w guitar amp with 4 el34s and 12ax7 preamp tubes. The transformers are not great quality I think, but they are not small.

Will this PT and OT work ok in a smaller, but quite different, build? I am thinking about something along the lines of eg an Ampeg b15 with 2 6l6 tubes. Could something like this work, or will the transformers intended for the above guitar amp be mismatched with a smaller design like suggested here? Like 2 6l6 or maybe even 2 6v6 tubes? Thanks!

[HotBluePlates]

I have a PT and an OT from a gutted 100w guitar amp ... Will this PT and OT work ok in a smaller, but quite different, build? ...

I have a 100Ω resistor.  Can I use it where a 100kΩ resistor is needed?

Kinda the same thing:  The OT can be "mis-loaded" on the secondary side to achieve a different primary impedance (within some limits), but mostly the OT and PT were "right-size" for the tubes and output power of the original amp.

[acheld]

Could something like this work, or will the transformers intended for the above guitar amp be mismatched ?

Yes you can make it work.   But . . .  just no.   

The PT circuit can be made to deliver the proper voltage and current by the creative use of dropping resistors, and if you pay close attention to your speaker impedance and match one up with your OT, then you're "ok."     

But you're really not ok, 'cause you pay the price with heavy components, excess heat, not optimized power supply.   Not worth it IMO, and yes of course I have tried this . . . 

[Erik Aa]

Thanks guys. So as I suspected there are mismatch issues. Quick follow up question. Would 4 6l6gc work (instead of 4 el34)? Or would this also lead to problematic mismatch issues? Thanks again.

[Esquirefreak]

4 6L6GCs will probably be just fine with that PT.

/Max

[sluckey]

Short answer... No! You cannot make a silk purse from a sow's ear.

A PT does not have a regulated output voltage. The output voltage depends on the load (mostly the power tubes). No load will have a high output voltage. A heavy load (such as 4 EL34s) will pull the PT voltage down quite a bit. A light load (such as 2 6V6s) will pull the PT voltage down only a little bit, maybe not even at all. The results will be a B+ voltage that is much too high and you will soon be crying, "Help, my B+ is too high!" The proper solution is to choose a PT that is matched for the intended load.

A basic good idea is to use the PT and OT in a configuration similar to what they were intended. 4xEL34s or 4x6L6s will make for a happier build.

[HotBluePlates]

... I have a PT and an OT from a gutted 100w guitar amp with 4 el34s and 12ax7 preamp tubes.

... Would 4 6l6gc work (instead of 4 el34)? ...

Yes, that should be fine.  Why?

The amp's power supply makes some amount of voltage available (say 450v), and the output tubes merely pull current through the OT's primary impedance (say 2kΩ).  How much current?

When the amp is at full-tilt, the tubes see a load that's 1/4 the full primary impedance:  2kΩ / 4 = 500Ω
447mA through 500Ω create a voltage-drop of 0.447 A x 500Ω = 223 volts
The 223v dropped times the 447mA of current equals 223v x 0.447A = ~100 watts

Both are RMS values:
  - 223v RMS implies 315v peak (plus some 100v or more across the tube), so 450v or more is sufficient.
  - 447mA RMS implies 632mA peak, split among 2 tubes (on that side of the push-pull output), or 316mA per tube.
  - The 6L6GC data sheet (top graph Page 6) says the 6L6 can manage 320mA peak with 400v (or more) on the screen.

We know the EL34 delivers around the same output power that the 6L6 delivers, based on decades of amps using 4x6L6 or 4xEL34 to make broadly the same output power.  Your only challenge is the EL34 doesn't require as large a bias voltage to tame its idle current, so you may have a challenge making a big enough bias voltage to keep 6L6s from overheating.  It depends the specifics of the PT.

[Ronquest]

Many post about, "I have a 100 watt Marshall or whatever and I'd like to pull 2 tubes, what do I need to do?"  Then a list of "change your output for your speakers, adjust your bias and check your voltages"  Now it can't and shouldn't be done?   We don't know the voltages of the PT or the OT's impedance ratio or even the cheap amp it came from.  The voltages could be on the edge for a 100 watt and without testing, we have no idea what the OT is.  Have you guys seen these cheap transformers?, half the size of normal and generally suited for half the rated power.
Now this doesn't mean I don't agree with the above post, the proper transformers are the best bet, just conversing, and I'd like to know the specs.

I don't know Erik Aa's abilities, but if he's asking then???

[WimWalther]

I have a 100Ω resistor.  Can I use it where a 100kΩ resistor is needed?

Kinda the same thing (...)

Sorry, but that qualifier "kinda" is doing more than its share of work. I don't understand how a possibly undersized xfmr is akin to a resistor that differs from spec by a full three orders of magnitude.

The former may or may not be up to the job, while the latter is totally unsuitable in virtually any situation.

[shooter]

You cannot make a silk purse from a sow's ear.

does that clear up your confusion?
kinda like the difference grow'n up city or rural
 

[HotBluePlates]