Help me understand this screen supply

[Duckmang]

I've got an old Grommes Precision PA amp that I'm repurposing to guitar amp duty.  It has a quad of 6GT5, 6C4 cathodyne inverter and an OA2 regulator in the screen supply.

Sometimes when I'm pushing it a little hard I see the OA2 dim and occasionally on heavy dimming the sound drops in an unpleasant way.  The more I look at this circuit the more confused I get.  The screen supply starts as a single lead from the power transformer that goes directly to a multi section can cap.  A high wattage resistor connects to another tap on a 2nd can cap.  From there to the OA2.  I get DC voltage on the lead directly from the transformer.  No diode or anything so this has me a bit confuzzled.  Can anybody explain what's going on there?

I was thinking that maybe the can caps are weak being original and old. I tied in an unused leg on a can cap but that led to a slow oscillation that you could hear and see in the OA2. The one time I missed the speaker connection, I noticed that the OA2 went dark when I tried to push a signal.  Will speaker ohmage mismatch cause this dimming/ dropout behavior?  Amp has a 6.25 tap and I'm running into a 4 ohm speaker. 

Any thoughts or ideas are appreciated.

[sluckey]

The more I look at this circuit the more confused I get.

Wish I could see it too!

[tubeswell]

If you want our help, please supply the schematic (instead of expecting us to spend hours of our lives that we won't get back trying to search for it to find out what you're talking about)


OA2 is a regulator tube. Have you tried subbing in a fresh one?

[kagliostro]

I can only guess It Is similar to this arrangement


But It isn't your schematic, so It will be better if you post It


Franco

[dwinstonwood]

Maybe this?

[Duckmang]

Its very similar to this last schematic but it doesn't have a long tailed pair.  My amp has a cathodyne so how the NFB ties in is a little different too.  I haven't traced every lead but that's the only noticible difference.   The power supply looks to be the same.   I haven't been able to find the exact schematic for this amp.

I don't have a spare OA2 to sub. 

[Duckmang]

Scratch that comparison, here's the closest schematic I can find.  The screen supply in this schematic doesn't show the filter cap connections.

[PRR]

Franco's #3 has no gas tube.

Dwinston #4 has gas tube regulating the G1 (not g2) bias.

Duck #6 has gas tube actually regulating G2. This might be "the OA2 dim and occasionally on heavy dimming the sound drops in an unpleasant way." But are we sure?

If so: realize that a PA amp might "WANT" to abruptly sound-like-crap to protect itself from abuse. Guitar amp design has to do something else. And I'm wondering if 6GT5 really belong in guitar amps. And how much work it would be to re-work for 6L6 types. AHHH sorry, the sockets are not even close.

[kagliostro]

I don't know if 6gt5 tubes Will perform well in a Guitar amp, that tube has a magnoval socket and so, as PRR say, no simple socket rewire to use 6L6 tubes

If you go to Doug's schematic database under Davoli Krundaal (an old italian brand) you can find some schematics with magnoval tubes (EL500 - EL504) as Power Tubes and people say they play good

Franco

[Duckmang]

I went ahead and plugged into the amp with it on the bench.  The OA2 regulates the screens at 155V.  The tap from the transformer has 220V.  There is a high watt 3K ohm (3.11k) between the two filter cap taps.  The 3K drops the input to the OA2 down to 157-158V.  When loading the amp pretty good there is a huge sag downstream of the 3K.  The ~157V sags as low as 69V but upstream of the 3K the 220V never sagged lower than 215V

With this set up the OA2 is only regulating a few volts and the screen current is severely bottlenecked. I'm very tempted to try to dig up a high wattage resistor somewhere in the neighborhood of less than 3K and sub it in. 

The outputs have 100 ohm screen resistors.  I'm also thinking it would be good to get those around 1K ohm.  The voltage drop across the 3K ohm in the filter network shows the quad's combined screen current at 21ma.  Am I correct in assuming that if I raise the screen resistors value, I can lower the 3K filter resistor's value to achieve the same/similar combined screen current and still be in the ballpark?  Any thoughts on that plan?

I still want to use the 6GT5.  Not only do they fit and get along with the OPT trans, I have a couple spares to boot.  They are also a really cool looking tube, the heaters have a really good glow. 

 

[kagliostro]

If you are looking for different arrangement around 6GT5 tube, give a look here

https://www.angelfire.com/vt/audio/f106c.html

Franco

[sluckey]

If you are looking for different arrangement around 6GT5 tube, give a look here

https://www.angelfire.com/vt/audio/f106c.html

That's the same schematic Duckmang posted.

[kagliostro]

Ooopsss


Franco

[HotBluePlates]

... The OA2 regulates the screens at 155V.  The tap from the transformer has 220V.  There is a high watt 3K ohm (3.11k) between the two filter cap taps.  The 3K drops the input to the OA2 down to 157-158V. ...

Apologies if you already know this...

An 0A2 is best seen as a "150v regulator" or "155v regulator."  Up to a limit (that kills the tube) it will pull whatever current it has to until its plate sits at ~155v.

It is used in a "shunt regulator circuit" that is in parallel with the load (here, output tube screens) and can draw more current to prevent the voltage from going higher.  This kind of shunt regulator can never prevent a voltage from falling below some limit or boost a too-low voltage, it can only pull a too-high voltage down.

The data sheet for the 6GT5 appears to imply that "150v" is the optimal screen voltage.  That's the only screen voltage for which there are curves, and the tube is only rated up to 220v on the screen (despite being able to handle high voltages on the plate).

It could be the designer intentionally wanted to clamp the screen at ~155v.


Since the 0A2 can only suck current, there is a resistance between it and a higher-voltage source.  The resistor drops voltage based on (total screen current + 0A2 current).  If tube screen current is small, that resistance might be pretty high.

The 6GT5 data sheet shows a 10:1 change of screen current when the tube is driven hard, implying 20:1 for a pair of tubes on one side of the output stage.  There's no chance a lowly 0A2 can not-pull that extra current to keep the screen node from sagging.

However, if you make the resistor too small, the 0A2 will kill itself trying to pull enough current to sag the voltage down to ~155v.  I think you're in a no-win, where the designer intended this amp to stay clean and never be pushed (a case where "too much screen sag" never happens).

... The OA2 regulates the screens at 155V.  The tap from the transformer has 220V.  ...  The 3K drops the input to the OA2 down to 157-158V.  ... I'm very tempted to try to dig up a high wattage resistor somewhere in the neighborhood of less than 3K and sub it in.  ...

220v - 155v = 65v
65v / 3kΩ = ~22mA for 4x screens + 0A2. 

4mA per screen?  So 8mA for 0A2?

If you make the 3kΩ resistor smaller, all that happens is the 0A2 sucks more current when the amp idles.  Unless you can figure out the split between the screens & 0A2, you run the risk of burning up the 0A2.

If you really want low-sag, you'll need a choke or a small resistor, and remove the 0A2 altogether.  6GT5 bias will need to be much, much cooler.  Dunno if it works without self-destruction.

... The outputs have 100 ohm screen resistors.  I'm also thinking it would be good to get those around 1K ohm. ...

Why?  That would increase screen voltage sag, and be exactly opposite what you want to accomplish.

The 6GT5 is a beam power sweep tube.  Screen current is relatively low, and screen damage is not likely (unless you really raise that screen voltage).  Low series-resistance is fine.

... The voltage drop across the 3K ohm in the filter network shows the quad's combined screen current at 21ma.  Am I correct in assuming that if I raise the screen resistors value, I can lower the 3K filter resistor's value ...

No to this, mostly explained above.

Screen current at idle ≠ screen current when driven (hard).  Screen current rises gently until the plate is pulled to some low voltage (right when you're trying to make max output power & distort).  Then screen current shoots up, causes the screen-sag & clamping/compression you've already heard.

Bigger series resistors just make that worse.  Smaller 3kΩ resistor just kills the 0A2 easier.  There may be a small "Goldilocks zone" where you can decrease the 3kΩ a little, but it won't make this amp easier to distort (it will always fall apart in that zone, unless you do a re-think of the power supply).

I don't know if the juice is worth the squeeze.

[Duckmang]

Hotblueplates thank you so much for that reply.     It clarified a lot of the things I wasn't up to speed on.  It seems that it'll be a challenge to make a truly good amp out of this guy, but I revel at the opportunity now that I better understand what I'm up against. 


 

[PRR]

> data sheet for the 6GT5 appears to imply that "150v" is the optimal screen voltage.

It may be optimal for *pulse* work and good life (studio TV monitors).

It will pass 390mA @150Vg2 which is way more than a 17.5W (even 35W) plate can use in linear audio service.

Another technique, possible today, is a big MOSFET pass device for screens. I have done that on 6L6.

[kagliostro]

Recently I opened a thread at DIYItalia about G2 PS Types for tubes that has G2 that works at a less Voltage than Plates

There are many way to drop voltage for G2

Doing the search to add different versions of PS I found also a stabilized PS by Pete Millet and, to me, it is very well planned

On the link all explanations, Board, Layout etc. etc.
http://www.pmillett.com/HV_reg.html



Here you can find the small collection, there is no discussion about this is better this is less better, only schematics to be seen to study various methods for feeding a G2 that requires less Voltage than the Plate Voltage

http://www.diyitalia.eu/viewtopic.php?f=22&t=11280

Franco

[Duckmang]

So I spent a little time playing around with this screen supply circuit.  Nothing I tried really made much of an improvement.  With the OA2 out, screens hotter, and bias colder the screen voltage was still being pulled really low.  Tried to add the OA2 back in via a diode if the screens fell below 155V but it wasn't very stable. 

Gave up and started reading when I came across a mention of the possibility of the screens supplying current if the voltage drops too low.  Lightbulb moment!  Maybe that's what's happening in my amp.  Reversed all of the mods back to stock screen circuit save for a diode in series between the screen grids and the OA2.  So far this seems to have eliminated the harsh cutout I've been experiencing.  Screen voltage is still dropping pretty hard but I don't care so much about voltage as I do about sound.  Since the amp isn't dropping out it is now apparent that the distorted channel gets rather "blatty" when cranked so I'll need to play around with the preamp section a bit.  I'm starting to get hopeful about this thing after all.