Choosing a vintage recicled PT for 5f11 project!

[angelothewolf]

Hi there.

I have this three trannyes for my Fender 5F11 project. I wonder if any is suitable for the job?



from left to right>

Original PT from a Gibson GA30RVT amp.

PRI: 120V
SEC: 6.3V-0-6.3V / 5V-0 / 350V-0-350V
something like this>


the middle one is from a nice vintage voltimeter HP400
PRI: 230V/115V
SEC: 0V-6.3V/ 0V-23V / 390V-0-390V



The last one is from a RCA Sideband Response analyser (more thant 20 tubes)
it is a 234KVA transformer
PRI: 0 - 105-115-125
SEC: 400V-0-400V / 0-6.3V / 0-6.3V / 0-5V
(couldnt find the schem for this one)




What do you think? can I recicle one of this?

[Tom Phillips]

Of those three choices I would use the PT from the Gibson amp. The PTs from the test instruments just don't have sufficient current supply capacity. However, the final B+ voltage may be a little high for your 5F11 with 6V6 power tubes but there are ways to deal with that if you decide to proceed. You could also consider building a low power PP 6L6 amp.
Tom

[angelothewolf]

Yep was what i think too, maybe the RCA has the current capability too... But what about that tap on the HT winding that goes to the "Depth" pot??

[angelothewolf]

. You could also consider building a low power PP 6L6 amp.
Tom

Maybe a 5E3 would be a better choice...

[Chuck H]

Yep was what i think too, maybe the RCA has the current capability too... But what about that tap on the HT winding that goes to the "Depth" pot??

That tap is just a convenient way to derive bias voltage. You can design a bias circuit without the tap and in fact that's what the GA30rvt does (because it, too is a fixed bias amp). Notice where the output of the depth pot terminates at the output tube bias resistors. Now look at the Gibson schem and follow that node at the bias resistors back to the bias supply and you'll see how THEY did it. You would need somewhat different circuit for the 5F11, but the general idea is the same.

[Chuck H]

Maybe a 5E3 would be a better choice...

Either amp can be made to work. Either is a good choice. As Tom said, for a 5F11 the voltages will be a bit high. Not catastrophic, just higher than the stock circuit. But that would also apply to a 5E3.

[bob p]

I like the idea of using the Gibson transformer for the 5F11 project. I think it's the most likely to provide you with results that will put you in the right ballpark. And like Tom said, it's not going to be difficult to deal with voltage corrections if it should turn out that you need them. But you may not need them -- 6V6, 6L6 or 7591 can handle the kind of plate voltages that you're likely to obtain, and if the voltages still turn out to be high there are things that can be done to adjust them.

One thing to consider is that it might be helpful to model the voltages that your iron would yield in your proposed circuit. If you can measure the No-Load Voltage on the secondary taps, as well as the resistance measurements across the transformer leads, you can plug those numbers into PSUD2 and model the power supply behavior before you commit to buying other parts. Personally, I end up spending most of my time researching data sheets, doing calculations and designing amps on paper, and a lot less time actually soldering them up.

[bob p]

as an example of your power tube options: the 5E4-A (6v6) and 5F4 (6L6) Supers are basically the same circuit, use your choice of power tubes and have B+ voltages higher than the 5F11. I think you have lots of options with that Gibson iron.

[DRH1958]

That tap is just a convenient way to derive bias voltage. You can design a bias circuit without the tap and in fact that's what the GA30rvt does (because it, too is a fixed bias amp). Notice where the output of the depth pot terminates at the output tube bias resistors. Now look at the Gibson schem and follow that node at the bias resistors back to the bias supply and you'll see how THEY did it. You would need somewhat different circuit for the 5F11, but the general idea is the same.

Hey Chuck, isn't that Depth control for varying the bias voltage as a function of a bias modulated tremolo? See the top half of the first 12AX7? Doesn't that function as the tremolo oscillator and the depth is the adjustment for the effect it has on bias voltage? That .25uF 400 volt cap couples the tremolo output signal to the 250k pot and it functions as a voltage divider for the -31 volt non-adjustable bias and the adjustment for tremolo depth. You can see the speed control just above it. It is a Vibrolux after all. Not a huge deal, just to clarify is all. Were it not a tremolo amp, it would indeed vary the bias voltage.

[angelothewolf]

as an example of your power tube options: the 5E4-A (6v6) and 5F4 (6L6) Supers are basically the same circuit, use your choice of power tubes and have B+ voltages higher than the 5F11. I think you have lots of options with that Gibson iron.

great! I personally like more the 5F11, I have the Gibson output tranny too! I think the only modification of the stock circuit would be the negative supply for the bias, 'cause I dont have that tap on the HT winding... I'll have to figure that out...

[bob p]

I think the only modification of the stock circuit would be the negative supply for the bias, 'cause I dont have that tap on the HT winding... I'll have to figure that out...

http://music-electronics-forum.com/t2545/

[bob p]

I think the only modification of the stock circuit would be the negative supply for the bias, 'cause I dont have that tap on the HT winding... I'll have to figure that out...

http://music-electronics-forum.com/t2545/

[Tom Phillips]

...I think the only modification of the stock circuit would be the negative supply for the bias, 'cause I dont have that tap on the HT winding... I'll have to figure that out...

You can read about various methods to derive a bias voltage from a transformer without the special bias tap at www.valvewizard.co.uk/bias.html

[DRH1958]

That Gibson had 7591 output tubes, so it's likely that OT has a primary impedance of 6.6k whereas 6V6s usually like to see 8k. I suppose it's not all that far off. Anyone else have thoughts on this?

You can also see how they did the bias for the later version of the Vibrolux, the 6G11-a where they didn't use that tap, just came off the main AC out. This could also give you a clue. You could make that 33k resistor a trimmer pot for adjustable bias: http://el34world.com/charts/Schemati...lux_6g11-a.pdf

[bob p]

That Gibson had 7591 output tubes, so it's likely that OT has a primary impedance of 6.6k whereas 6V6s usually like to see 8k. I suppose it's not all that far off. Anyone else have thoughts on this?

Well, yes. One of the reasons that I recommended modelling the PSU circuit with various tube configurations was to determine which tube compliment would work best before buying ohter parts... like output transformers. 6L6 would be at the low end of the range for plate-plate impedance, 7591 in the middle, and 6V6 at the high end of those we've discussed. But if the deluxe reverb has shown us anything, it's that you can make both 6L6 and 6V6 work with a 6k OT.