Another Transformer Question

[Shadrock2]

So a couple weeks ago I asked about transformer inductance for a power supply I was making for a gibson stereo amp(gibson_ga-79rvt). The transformer seems to work ok.

I constructed a circuit as shown in the power supply section axcept using a 575 ohm 20W resistor instead of the inductor and using a virtual ground for the 6 volt heater with a pair of 100 ohm 1 watt resistors. This resulted in 401V B+ with a 27K load resistor (10 - 15 ma). I feel like this is way too high for the 6BQ5 output tubes.

So I got this hairbrained idea and wired the 5V winding in series with primary winding to add turns to the primary. I used my inductance meter to determine correct polarity. This reduced the output B+ to 389V which is better.

Is there any reason wiring the transformer like this is a bad idea?
The 6V heater winding was also on the high side.

Also how much current do you think this amp will draw total?

[Helmholtz]

Is there any reason wiring the transformer like this is a bad idea?

I don't think so. But as you have a 5V winding, have you considered to add a rectifier tube (5U4) to bring voltages down?
My main concern would be screen voltages considerably exceeding the 300V limit.

[Chuck H]

I like Helmholtz's idea of adding a rectifier tube. Though that may not be the easiest solution. Otherwise I don't think there's a problem with what you've done. But then...

I don't think there's a problem for el84's with a B+ 400V when loaded at just 12.5mA. The actual loaded current with all tubes in place will be more like 120mA to 130mA at idle. So that's going to load it down a lot more (I don't know the winding resistance). I'll guess you'd end up with about 380Vp. That's not unheard of for el84's in guitar amps at all.

Helmholtz is always looking out for the poor screen grids (somebody has to ) so why not use a 1k resistance in place of the choke instead of 575r. My own amp (2xel84) has 365Vp and I use a 2.2k resistor there. So that would be about the same as a 1k for your application and I think you could expect a screen voltage around 355V. That's not at all unheard of in guitar amps either. and IMHE the screens will be fine.

I'm using 470r screen grid resistors. I know that 1k for screen grids is the new norm, but the amp sounds a lot snappier and tight with 470r. So since the tubes seem to last just fine in this amp (and it lives dimed up) I'm staying with 470r. You should go with 1k screen grid resistors, but if it feels flat and listless you might be ok to try 470r, BUT, only if each OT primary impedance is 8k or less. If you have 10k primary impedance you better stick with 1k for the screen grids.

El84's are TOUGH. I've said before that they're the bitches of the guitar amp world and I meant it. They get treated horribly in many commercial designs and have been for half a century. Yet you hear less about this tube failing catastrophically than any other I think. It's just a tough tube and I think you'll be fine without the weird series connection at the PT primary.

You can always reduce filament voltage with a couple of low value resistors. Since I use Hammond transformers I've done that a lot. ***Hammond use to use a 115V primary winding and tend to be generously rated WRT filament current. My experience is that they always run 8% to 9% high with 120V at the primary.

[Helmholtz]

I'll guess you'd end up with about 380Vp.

And this will be reduced by the cathode voltage of around 14V. But the amp will need cathode resistors to be adjusted to not exceed 100% dissipation.

I expect a total DC current of 150mA at idle.

[Helmholtz]

Helmholtz is always looking out for the poor screen grids (somebody has to )

Well, I don't like having to replace tubes after 6 months operation .

EL84s may take some electrical abuse, but the problem is often mechanical (rattling) caused by high temperature/overdissipation.

[Mick Bailey]

Many Traynors, the Marshall 20/20, Mesa and plenty of others run EL84s at anything up to 450v or even more. There are Russian tubes that survive well over that and my favourites are the ones Sovtek branded up EL84M. Now they're rather more expensive than I like. Generally JJs last OK and like Chuck I always run 2k2 screen resistors.

I have one hangup about using a secondary winding on the mains side as a voltage buck, but it depends on the particular transformer. If it's a fully isolated bobbin it defeats a safety aspect of the transformer that was there by design, in that the isolation is now compromised. A short between the windings can place mains voltage on the secondary. The fuse would blow, but for a moment I wouldn't want to be on the receiving end.

[Helmholtz]

I have one hangup about using a secondary winding on the mains side as a voltage buck

You are completely right. I misread and thought it was used on the secondary without checking the voltage values.

[Chuck H]

Well, I don't like having to replace tubes after 6 months operation .

I use to have that problem with el84's in two amps that were at or over 400Vp and used a choke to the screens (so they were only a couple of volts lower). But not how you're thinking. These were both combo amps and the tubes would get so beat up physically that they'd start coughing and jangling after a few months. My own amp has a high-ish Vp, but not THAT high, so with the slightly lower output I would get about six months. I've since switched to the Rusky 6p14p-K (vibration resistant) tube for that amp and haven't had any trouble in roughly twice the service hours as the standard market stuff.

P.S. I still don't know what the proper moniker for that tube is in the latin alphabet. I see it marked as 6p14p and 6n14n interchangeably.?.

[Shadrock2]

This amp uses a remote power supply located in the bottom of the combo cab. I am reconstructing the power supply from scratch and most importantly for me... on the cheap. The amp used an octal socket to transfer the heaters, B+, and power switch to the main amp chassis. The chassis I used for the supply uses that octal socket to fit the X-fer plug. Also that chassis was cut approximately in half to fit in the cab and there is not much room left for a tube.

Mick gets at what I was wondering about. And if the rest of you think I can run this at > 375V then maybe I won't worry about the extra volts. I was thinking of adding some screen resistors anyway. But I really don't want to ruin the input tubes (6EU7's and a 7199). I was going to start by replacing the cathode bypass caps but maybe I should replace the sandstone like cathode resistors also?

[Mick Bailey]

For a long time now I've been using a simple MOSFET B+ reducer in all kinds of amps. It requires one hole to mount on the chassis, is discreet and is fit and forget. You can decide exactly what your B+ needs to be. I have one particular problem amp, a Dr Z Maz 18 that's used by a pro player and tubes never lasted more than a few months until the B+ was reduced.

EDIT: see post #5 here:
https://music-electronics-forum.com/...ad.php?t=39329

That's exactly how I make 'em.

[Dave H]

P.S. I still don't know what the proper moniker for that tube is in the latin alphabet. I see it marked as 6p14p and 6n14n interchangeably.?.

It's a letter from the Cyrillic script often translated as 'p' but it looks more like an 'n' with a flat top or the symbol for pi

[Enzo]

It looks like an N, but is pronounced as P and serves the place of P in use.

Their N looks like our H.

The Russian "ресторан" reads restoran in English.

[The Dude]

And this will be reduced by the cathode voltage of around 14V. But the amp will need cathode resistors to be adjusted to not exceed 100% dissipation.

I expect a total DC current of 150mA at idle.

^^^^^^ That. Don't forget that the plate voltage rating is plate to cathode- not ground. The tube doesn't know if it's cathode is grounded or not.