5F4, Plates lower than screens, 1k's on the screens

[satsloader]

Hi all,

First fire-up, all looks normal, except that I'm seeing 3-4vdc less on the plates than the screens. It's got 1k's on the screens. I then added a 100ohm/7watt to feed the 1k's ( 100ohm between the 2nd filter cap and 1k's ), little if any change. I then moved the plate feed to the 1st filter cap ( 1.5H/300ma choke between the 1st and 2nd caps, 42 ohms resistance ), no real change.

B+ at 1st filter cap is 385vdc, normal for this power transformer and load, nos RCA 5881's. In fact all the voltages are normal. Plates are approx. 374vdc, screens are usually 3-4vdc higher.

Note; the OT is the Classictone 5F4/5E7 28watt, with 198ohm's resistance one leg to CT, 185ohm's from the other leg to CT.

I tried a pair of nos 6L6's in place of the 5881's and they have the same results, 3-4 higher screen voltages.

Previously I've had a 5F6-A in this chassis with the same PT, and a 420vdc B+ PT as well. It used a Bassman OT ( leg resistances in the 150's ), and a 5H/90ma/250ohm choke. I used 470ohm screen resistors in that build, and the screens were always a volt or two lower than the plates.

I'm having a hard time imagining that the higher resistance of the Classictone OT ( compared to other OT's like the Bassman ) is pulling the plates below the screens, given the 100ohm branching off the 2nd filter cap, then feeding the 1k's/screens.

I removed the phase inverter tube to see if an oscillation in the pre/PI was possibly causing the lower plate voltage. It didn't change anything.

I should mention that the amp sounds normal; Clean up to 9 on either volume. I expected it to be louder than it is, even with the B+ on the low side for 6L6 types, at around 375vdc ( wall a/c supply has always been 120vac, pretty steady too ). The amp is louder than my Silverface Champ, but not as loud as it was with the 5F6-A circuit, even using the same PT and speakers.

I have a Tektronix dual scope and a signal generator. Should I go looking for an oscillation in the power section? Wouldn't an oscillation also pull the screens down?

Any suggestions?

Sincerely,

Bill in Seaside Ca.

[eschertron]

So a voltage drop across the primary legs of 10vdc / primary leg resistance of ~200R equals a idle current of 50ma. Is that reasonable? If so, there's no problem. I don't suspect HF osc or anything else unnatural.

If I were concerned about the screen voltage, I might put a bigger resistor between the B+ node and the screen node, but that's going to decrease the audio power of the amp even more. But I'm not concerned, because under load, the 1k screen resistors will start to drop a few volts (20vdc at 20ma screen current, for example) and that will bring the screens lower than the plates.

[Enzo]

I'd consider this condition normal really. The stock amp has the screens and OT CT connected together. Under those conditions, the 190 ohms or so of the transformer can't help but lower those plates a few volts.

Moving the plate supply to the first cap means it is less filtered, and may result in increased output stage hum.

You stuck a 100 ohm resistor in there. The schematic shows the small tubes all add up to about 7.8ma B+ current. If that flows through a 100 ohm resistance, it drops less than a volt. Your two screens probably are not drawing that much, so they won't drop much voltage across that resistor either. You added 1k screen resistors? Each milliamp of screen current then drops 1v across the 1k. So how much sscreen current are your tubes pulling?

[Chuck H]

FWIW I've seen the issue of 5f4 screen voltages being higher than the plates maybe a dozen times here over the years. It's not an indication of anything being wrong. It's just the nature of the design. The general consensus seems to be not to worry too much about it. For the sake of modern tubes (in case they actually aren't as robust as NOS tubes?) the 1k screen resistors are probably a good idea. Since the choke has considerably lower resistance than the OT it doesn't surprise me that the problem remained after moving the plates to the first filter cap. Enzo mentioned the risk of more hum. I've simulated some older amps on Duncan PSUD and I must say that the ripple with a single small (relatively) value cap is pretty awful. If you're output tubes are matched under all conditions it may not matter. But that's a perfect world. There's a good chance things will match less under dynamic conditions and the result would be ghost notes or even a mild ring modulator sort of tone. This would be most noticeable when clipping. I'd probably put a 33uf in as the first cap or just parallel another 16uf there.

[satsloader]

Hi Eschertron and Enzo,

The plate currents are 35, 38ma. Read with a Weber 2nd version ( big box ) bias meter

I'll have to measure the drop across the 1k's and calculate the screen current. First fire-up was with the plate and screen supplies both off the 2nd filter cap, with the 1k screen resistors installed across pins 1 and 4 ( I couldn't imagine running without something on the screens.. ). I added the 100ohm to see if it would help some. Since I've moved the plate supply to the 1st filter cap I thought I'd swap the 1.5H/300ma/42ohm choke now in it with the 5H/90ma/250ohm choke and remove the 100ohm. I'll probably eventually install the bigger PT as well to bump the volume back up. there was a very small amount of 120hz hum increase with the plate supply moved to the 1st cap, but overall the hum is very quiet. Not much more with tubes than without ( While I was conditioning the filter caps before installing tubes I noticed a smidge of 120hz, assumed it was inductive coupling from the traditional Tx/choke placement ).

Thanx for the fast replies,

Bill

[satsloader]

Hi Chuck,

Thanx for the input, I'm probably just being paranoid. I assumed a volt or two difference would occur, but was nervous seeing 4 or 5vdc difference. With your input, I'll leave the big choke in and move the plate supply back to the 2nd cap, dump the 100ohm, keep the 1k's, install the bigger PT, and see how things develop.

Cheers,

Bill

[Enzo]

I appreciate that you couldn't imagine running th amp without something on the screens, yet there are thousands of these amps that have been running exactly that way the last 50 plus years without issue. There is how things ought to be, and then there is real life.

OK, 35ma and 190 ohms, then your plates will be about 6.6v lower than the B+.

it is so simple to use Ohm's Law when you have the currents - or can easily calculate them - and resistances. Rather than guessing at 100 ohms r whatever, if you really want to drop your screens, then calculate what resistance you do need.

A volt or two or four or five volts, no matter. At even 400v, 4v is only a 1% difference. never lose sight of the fact these circuits are just guitar amps, not precision electronics. Something to help perspective: with 375v B+, for every volt your 120v mains shifts, that B+ will shift over three times as much. Your mains hop around all the time. A 3v rise in mains, and your B+ went up 10v.

[satsloader]

Thanx Enzo.

[pdf64]

If you want to pull the screens down to / lower than the plate V under static conditions, then consider adding a resistor between the screen grid and ground, eg 100k =>2W.
That would pull another 3-4mA through the 1k screen grid stopper, dropping 3-4V.
I think Andy LeBlanc came up with this idea over on TAG, he's convinced it has tonal benefit.