Fender Princeton very high B+

[BlacknWhite]

Hey guy's just wanted to see if someone familiar with the old Tweed princeton amps could answer this. I'm use to working on older vintage gear and the higher B+ voltages but This is the first Tweed era Fender amp. I'm working on a 57 princeton 5F2-A. It came in with no output and very high voltages. I quickly found it had been mis wired during a cap job and the cathode and bypass cap were not grounded. I corrected that and now have output but the voltages are all very high still. The first node after the rectifier is at 445 vdc and the plate of the 6V6 is at 440v biased just under 12 watts. I know back in the day normal voltage was around 300-3 10. Do these amps run this hot at 120± line voltage?

 

[DADGAD]

Are you using a 5Y3 rectifier? What does your line voltage measure? What does your filament voltage measure? If you have a VARIAC, you can dial back the input voltage until filament voltage is 6.3 VAC. Then remeasure the B+ to see what the amp was designed to put out.

In the end, you may need to build one of those bucking boxes to lower the input voltage.

 

[Lowerleftcoast]

What brand and type of rectifier tube is in this puppy? Several new production rectifier tubes do not drop voltage like the vintage.

What VAC do you meter on the PT HT?

Check the PT CT for good connection.

 

[BlacknWhite]

Yeah it's a 5y3. Line voltage is 122 ish. I had to dial my variac down to like 90 volts to get it close to original specs. I've had amps where I had to bring down to like 110/115 but never had to dial it down to 90v haha

 

[BlacknWhite]

What brand and type of rectifier tube is in this puppy? Several new production rectifier tubes do not drop voltage like the vintage.

What VAC do you meter on the PT HT?

Check the PT CT for good connection.

I'm at work now but I think it's just a JJ 5y3 in there. I'll double check the Vac at the transformer tonight. I checked all the grounds and they were all good.

 

[Phrygian77]

Are you absolutely sure the output tube is drawing current, and that the B+ isn't inflated because there's practically no load?

 

[Lowerleftcoast]

The JJ 5Y3 usually drops the voltage as much as a vintage 5Y3.

To satisfy the question of practically no load, please provide a voltage chart.

 

[BlacknWhite]

Are you absolutely sure the output tube is drawing current, and that the B+ isn't inflated because there's practically no load?

Yeah I have a bias probe on the 6v6 while I dial it up on the variac and it is drawing current.

 

[Commodore 64]

Can you measure the standing current in the circuit?

 

[schmee]

I dont know the tweeds, but the BF Princetons run very high like that...

 

[BlacknWhite]

Can you measure the standing current in the circuit?

My variac read out shows just under 300mA is that what you are referring too?

 

[Phrygian77]

Yeah I have a bias probe on the 6v6 while I dial it up on the variac and it is drawing current.

In your first post, you say 12 watts at 440. Did you change the cathode resistor? Regardless of whether or not you want to exceed the 6V6's max dissipation, that's not a hot enough bias at that voltage. The load line shifts with the higher plate voltage causing more cutoff clipping if you try to keep the bias low.

 

[BlacknWhite]

In your first post, you say 12 watts at 440. Did you change the cathode resistor? Regardless of whether or not you want to exceed the 6V6's max dissipation, that's not a hot enough bias at that voltage. The load line shifts with the higher plate voltage causing more cutoff clipping if you try to keep the bias low.

That's with the 470 ohm resistor. I was getting 26mA at 436 plate voltage.

 

[D'tar]

26mA suggests your 470 has drifted, IMO

Did you measure the actual resistance and what is the cathode voltage?

 

[Wally]

Voltage chart is needed...as noted above. What is the cathode voltage on that 6V6? How did you arrive at the current time draw number? Is the power transformer original?

 

[BlacknWhite]

Voltage chart is needed...as noted above. What is the cathode voltage on that 6V6? How did you arrive at the current time draw number? Is the power transformer original?

I'm at work right now. I'll post all my numbers tonight when I get home.

 

[dan40]

It's odd that the variac had to dial down to 90vac to achieve correct filament voltage. Is this the original PT and does the primary have multiple voltage options for different countries? If it's a replacement it may be accidentally wired to a 100v primary.

 

[BlacknWhite]

Got busy and just got back to the amp. Here is a schematic and I wrote the voltages on it. I made sure everything is to spec and filter and coupling caps have been replaced and tested. Tubes have been tested as well. Disregard previous readings as my DMM battery was low. These are the current readings. I tried increasing the bias resistor but the plate voltage was getting too high. The 5Y3 is actually a Sovtek it was hard to read. I'm going to either grab a JJ 5y3 or find a Nos one. Other than that looks like I'll be looking for the best way to try and drop B+ in this circuit.

 

[Wally]

What are the 5VAC and the 6.3VAC windings giving you?

 

[BlacknWhite]

What are the 5VAC and the 6.3VAC windings giving you?

I forgot 5v winding... It was 5.3v and the 6.3 was also 6.3 dead on.