Voltage Too high for mixed 6v6/el84 ??

[allsavy]

Hi I'm reworking an old PP 6v6 (pine pepco) and thought I'd convert to a tube rectifier for the sake of having that experiance. I savlaged a PT with the required 5v tap from an old spartan radio. Apparently it is wound for higher voltage than what is usually seen in guitar amps. AC voltage measurements at the rect socket are 366v ac @ pin 4, 336v ac @pin 6, 5v @ pin 8 and 470v dc @ pin 2. There are no other tubes in the amp as yet. It will host a pair of 6v6 and a pair of el84 in parallel PP with two 12ax7s in the preamp.
Even though voltages will read higher than with tubes in, it looks like I'll have to drop over 100v to use this PT. Perhaps more for the sake of the el84s ?

Install a big choke ?, big resistor ?, Zenner string?, mosfet? Light bulb ???? what?

Tear out the tube rect and PT and put the orig PT with ss rect back in?

[zhyla]

Zener would work... I've got a 100V/50W zener that was like $12 from Mouser. It worked fine but they do get hot.

[mooreamps]

Hi I'm reworking an old PP 6v6 (pine pepco) and thought I'd convert to a tube rectifier for the sake of having that experiance. I savlaged a PT with the required 5v tap from an old spartan radio. Apparently it is wound for higher voltage than what is usually seen in guitar amps. AC voltage measurements at the rect socket are 366v ac @ pin 4, 336v ac @pin 6, 5v @ pin 8 and 470v dc @ pin 2. There are no other tubes in the amp as yet. It will host a pair of 6v6 and a pair of el84 in parallel PP with two 12ax7s in the preamp.
Even though voltages will read higher than with tubes in, it looks like I'll have to drop over 100v to use this PT. Perhaps more for the sake of the el84s ?

Install a big choke ?, big resistor ?, Zenner string?, mosfet? Light bulb ???? what?

Tear out the tube rect and PT and put the orig PT with ss rect back in?

Ya, but if you have that power transformer providing plate power for 4 power tubes, it's possible just the load of 4 power tubes would pull the High Voltage down about that much anyway. You will really need to measure the High Voltage under a full load before trying to figure out how much more voltage you will need to drop. I'm just guessing you have not done that yet. . .

-g

[allsavy]

I was hoping to get an educated guess before wireing everything up.
Givin the tubes mentioned, there must be an estimate-able voltage drop.
Even if the voltage drops 100 volts (probably less) I'm thinking I might increase the 1k 5w screen grid resistors to the el84s. What if this value needs to be 10k or more... would that interfere with the operation of the tubes? Usually screen grid resistors are 1K or sometimes less.

[mooreamps]

I was hoping to get an educated guess before wireing everything up.
Givin the tubes mentioned, there must be an estimate-able voltage drop.
Even if the voltage drops 100 volts (probably less) I'm thinking I might increase the 1k 5w screen grid resistors to the el84s. What if this value needs to be 10k or more... would that interfere with the operation of the tubes? Usually screen grid resistors are 1K or sometimes less.

Yes, there is. I will need to know :

1. what the current rating is on that power transformer.
2. class of operation on the PA ; {class A or class A/B}


Secondly, increasing the value of the SGR has little effect on plate current, and thus little effect on plate voltage. But, as a general rule, the more you load down the power supply, the more the output voltage will sag. Thus, running the PA in class A will present more of a current load on the power supply verses running the PA class A/B.

So, when we determine what the current load is, and what the power supply can source for current, will give us some idea of what we call output voltage under a full load, verses "no load" which is what you are measuring now.

Under a "full load" should be ; the no load value times 0.707. But we don't know the value of "full load" until we understand what the current spec is on this power transformer

Also, by knowing what the total current load is, we can calculate the values for any number of series resistors to drop additional volts.


{I'm pretty sure all this is right . . .}
-g

[allsavy]

I have no reliable way of knowing the exact specs of the PT. It was salvaged from a 1950s radio. I would assume it has a slightly higher current than what is typically used in guitar amps givin that radio circuitry is seemingly more complex. I hope it can supply enough to do what I need.
Although the amp will be cathode biased I assume class ab opperation.-
Guess I will wire things up and see. My concern is for the el84s. Will address that later if needed I suppose.

[Bruce / Mission Amps]

Without going back and rereading everything....did you mention qty and what power and preamp tubes were in the radio in the first place?

A dirty test:
Go buy yourself at least two 25 watt, 2500 ohm (or something close to that) power resistor.
Put the two in series by twisting the leads of one end of each resistor together and solder them so you have 5000 ohms at about 50 watts.
Get some clip leads and clip one on each end of the now series connected resistors.
Clip one end to the negative end of your first filter cap (ground) and the other to your B+ output from your first filter cap.
Fire up the PT.
Quickly measure the new B+ voltage under that 5000 ohm load.
Now divide the new voltage by 5000 and that will tell you how much current is flowing through the resistor.
If, instead of the 470vdc, you now measure 425v across the 5000 ohm resistor then:
425/5000 = about 85ma of current and, 85ma^2 x 5000 = 36 watts.
Yes, the two resistors will get hot as hell but you can monitor the voltages of the PT under load and the temperature of the PT and see how it fairs now.
If the B+ drop less then 10% it might be fine for a +25watt guitar amp with the right power tubes.
If the PT gets too hot to touch in 10 or 15 mins I'd say the PT is a light weight unusable guitar amp PT.
Or if the B+ drop dramatically, or the PT can't even hold over 420 under that 85ma load I'd think the PT is too light weight for a real 20 watt amp... or, just come back here and report what you find and others can chime in on what they think.

[allsavy]

Thanks for the tips!
Oh Bruce, while I've got your attention, are there any sound clips online of your tweedy deluxe. I'm not all that familiar with vintage amp tones and would like to hear an example of the characteristic sound of your design.
Thanks.
savy

[Bruce / Mission Amps]

Thanks for the tips!
Oh Bruce, while I've got your attention, are there any sound clips online of your tweedy deluxe. I'm not all that familiar with vintage amp tones and would like to hear an example of the characteristic sound of your design.
Thanks.
savy

Not mine but here's a fun link:

http://mr335.tv/index.html?channel=r.../rig/tweaktone