Soft start voltage descepancy

[johnsonad]

I'm having a problem with the soft start board not reaching proper voltage on the OB side of the 300k Ohm resistor. With the pot maxed I can get a max voltage of 119v.  This is a modified Paramount circuit and I'm not sure how much that plays into it or where to set it at. Should I use the A2 voltage/3 as the 2A3 uses?  The amp is designed around a 300B OP of 350v, 50mA, 5k, -75v. It seems like the 300B is oscillating  just a little (0.2v plus or minus at A1/A4).  Here are my voltages:

1: 429
5: 74
6: 74
9: 119
10: 0
11: 232
12: 164
13: 0
14: 28
16: 448
17: 0
18: 0
19: 117

A1: 77.5 to 77.7 (oscillating)
A2: 427
A3: 0
A4: 72.8-73.0 (oscillating)

B1: 3.09
B2: 1.9
B3: 0
B4: 119
B5: 0
B6: 300
B7: 0
B8: 7.1
B9: 3.09

Thanks,

Aaron

[Paul Birkeland]

This is a modified Paramount circuit and I'm not sure how much that plays into it or where to set it at.

You'll have to actually say what you modified so we can help you.  I'd guess that you have two identical Paramounts in this configuration?  Do they both exhibit the same behavior? 

Oscillation on the filament of the 300B could be any number of things.  The first thing I would try is to short the 249K grid to ground resistor on the 300B to see if that clears it up.

-PB

[johnsonad]

Hi Paul. Building the first and testing it before starting on the second. PS as discussed in a different thread using two PC-2 in series CLC with standard Paramount cap values. Stock soft start build. 5k EXO-050 OPT, Magnequest high nickel plate choke in place of the 249k resistor, Dowdy 80 Henry 60mA plate choke, 6.5uF PF cap, 1.5k cathode resistor, 100uF cathode bypass capacitor. 

Otherwise stock wiring configuration to the Paramount manual as far as terminals go.

I re-wetted all of the soft start pads with zero increase to voltage with the pot.

[Paul Birkeland]

What voltage ranges do you get across the 300K resistor when you turn the pot?

So you are using the 5670 with the PCB?

[johnsonad]

104.3v-117.1v, stock 5670 tube (I've tried two so far)

[Paul Birkeland]

How about the IA/OA voltage on the board?  (and the kreg voltage on the side w/o the trim pot)

[johnsonad]

IA: 453v
OA: 301v
Kreg (b side): 7.1v
Kreg (a side): 2.1v

[Paul Birkeland]

IA: 453v
OA: 301v
Kreg (b side): 7.1v
Kreg (a side): 2.1v

OK, the shunt regulator is working, which is great, but the bias trim TL431 is not.  In this configuration, the minimum bias voltage available is 2.5V, and max ~7-ish.

In terms of reasons that this wouldn't be working, I can think of the following:

1.  The grid of the driver 5670 does not have a grid to ground resistor present (249K in the Paramount).
2.  There is a funky solder joint in the biasing circuit.
3.  There is a funky solder joint in the B side C4S (most likely).

You can test the C4S by removing the A side Kreg feed to the 5670.  Since we want 4V of bias with 4.3mA of current, you can use a 1K resistor from the driver stage cathode to ground, then retest to see if your driver plate voltage comes up.  If it doesn't, you have an issue with your B-side C4S wiring/soldering.  If it does, then you need to reheat your joints in the TL431 biasing network, and be triple sure that you have all the jumpers that you need.

[johnsonad]

Thanks Paul. I removed the board and replaced it with the second board and it works as it should now.  Next question, what should the driver voltage be set at? 175v per the Paramount manual or 1/3 the voltage at A2?

I still have the oscillation problem. I replaced the grid choke with a 249k resistor and measured, same thing. I Then grounded the grid side of the resistor, still oscillating. The B+ oscillates all the way back to the PS output about a volt. The 300B filament supply and driver tube supply both oscillate but only by 0.05v to 0.2v.  Sounds like my PS? Of note, it's a slow oscillation. I tried increasing the coupling cap with no resolve.
The PS is 135uF (1.2 Ohm)/48H (660 Ohm)/ 135uF (1.2 Ohm)

[Paul Joppa]

PB's test is a good one - more data always helps!

Check that the trim pot (on the bad board) is the correct value. (Just  to be sure.) Also the 4.99K resistor R3 on the A side - it's different from the 2.49K on the B side.

More data on the oscillation would probably help. What is the actual evidence, and what is the frequency or period? Does it appear at the hum pot center tap? If so, what is the cathode resistor bypass capacitor (so we can guess why it isn't shorting the fluctuation to ground)?

[johnsonad]

Hi Paul,

I'll check the values on the board and report back.


I may be mixing my words and I'm probably blowing this out of proportion.... It doesn't oscillate at a frequency that you would measure, which means it's not oscillating..... What I'm measuring is that the voltages won't stay still. They keep fluctuating but when it's warmed up for a while it's at such a low level that I doubt it will make a difference.
I'm using a 10 turn hum pot with CCW to A4, CW to A1 and SW to 5U. The voltage at SW moves from 75.06v to 75.12v. The cathode resistor bypass capacitor is a 100uF film cap.

PJ or PB, what would you recommend I set the driver voltage at?

I ran a sine wave though it and ended up with 6.8v before clipping, 1.7v input for a whole 5.8 watts.

[Paul Joppa]

If it goes all the way back to the high-voltage power supply, then it's probably power line fluctuations.

Set the driver plate voltage to 175v; that gives the most generous compromise between headroom and compliance.

[johnsonad]

Both PB and PJ, thank you. I'll start on the second amp and work on that board.

[johnsonad]

Back again.

I built the second amp today and I'm still having problems with the soft start board. I have so far re wetted all of the solder points and verified that all of then jumpers and resistors are in the correct places.  Next I'll try and figure out the tests that PB suggested.

I'm running into another issue with this amp. At terminal 6 (standard Paramount wiring), I'm getting 75v DC. I tried a different PF capacitor with the same results.  Per the manual it should be zero.  I went back and checked and the original amp reads the same though I have zero writing down in my notes. Hmmmmm

[Paul Birkeland]

I'm running into another issue with this amp. At terminal 6 (standard Paramount wiring), I'm getting 75v DC. I tried a different PF capacitor with the same results.  Per the manual it should be zero

One side of the parallel feed cap is the 300B plate (~450V or so) and the other side is connected to the output transformer primary, which is connected to the 300B filament (~60-75V).

There is no specification for voltage at terminal 6 in the current manual, but your voltage is correct.