Lowering plate voltage on Fender amp

[Sredna]

Ok, so Im sitting here with a Fender Studio Bass amp and I'm getting ~530V plate voltage (should be 500V) and my cheap chinese 6L6GCs have great problems with that. 

It seems that the power transformer is already set to its highest input voltage, and that was probably sufficient in 1980 with 220V,  today we have 230-240 here in Germany.

The 39K/2W resistors (see attached schematic) direct after the rectifier bridge measure 43 & 47K.
It's slightly too high, could that be a part of the problem?

Any ideas out there how to get down the plate voltage?

Use lower values on the 39K resistors, Zener string from bridge minus to ground?

I have a couple of old Fender 6L6GCs (STR 387) who doesn't mind the 530 volts but this amp needs 6 of them and I'd rather look for cheaper altrenatives + be kind to the caps as well...  8)

Thanks in advance!

 

[pucho812]

what 6l6's do you have in there now?

 

[Sredna]

I have trown in what I had in the closet: 2 Fender STR387, 2 RSD (old east german) and the 2 new chinese ones.

As I bought the amp in non functioning order, I'd rather test it with old/cheap tubes before I order a whole new set.

 

[moamps]

It seems that the power transformer is already set to its highest input voltage, and that was probably sufficient in 1980 with 220V,  today we have 230-240 here in Germany.
The 39K/2W resistors (see attached schematic) direct after the rectifier bridge measure 43 & 47K.
It's slightly too high, could that be a part of the problem?
Any ideas out there how to get down the plate voltage?

The resistors 39k are just bleeder resistors, you can't regulate the  output voltage by changing them. There are two solutions, IMO:
You may insert one or two RC networks in HT power line,
or insert a small power transformer with 24V secondary inserted in series with HT secondary with flipped polarity.   

 

[Sredna]

Ok, thanks!

I'll try with the RC network as I'm not shure about how to dimension a subtractive transformer.

 

[ln76d]

You can try russian 6П3С-ЕВ (military version only -"ЕВ") which common are cheaper than good 6l6 and can work long even with 700V.

 

[Sredna]

Aha, I cant find much information on these, how would you name them with latin letters, 6P3S-EV?

How much heater current do they need?

Thanks!

 

[ln76d]

Aha, I cant find much information on these, how would you name them with latin letters, 6P3S-EV?

How much heater current do they need?

Thanks!

Sorry, my fault it is 6П3С-Е ("ЕВ" is for preamp tubes) so it should be 6p3S-E

http://www.tubes.ru/techinfo/HiFiAudio/6p3s.html

Don't worry about the max 250V - it's to keep on a "paper" higher life time.
With 700V it will probably not work for 5000 hours, but they can live very long.
For example in my firend ampeg v4b one set was used for few years instead of 7027A (540V).

 

[Sredna]

Thank you!


Another thought:

The screen (g2) voltages is as high as the plate (530V), 6L6GC is
specified for only 450V on g2, that means 80V over max rating!

(The OT has taps but not the regular ultralinear style (~40%) but at just about 12%)

Any way to reduce g2 voltage?

 

[CJ]

you can run the tubes at 530 if you drop the screen voltage,

use a sandblock pwr resistor, maybe 2K 10W to start with, try to knock off about 50 volts and then set bias to idle at 30-35 ma,
(per tube)

tube rectifier will also work but they do not like hi current high voltage apps so you have to replace often,

short outs are fun to watch, all those sparks and stuff,

 

[Sredna]

Great, thanks!





For sparks and stuff, I'll rather wait 'til new years eve...

 

[PRR]

Forget the plate and screen voltages.

Fender often ran tubes far past the factory specs.

And a true 6L6GC is known to be a high-voltage TV tube sold under an audio-familiar type-name, with published ratings set low so that HIGH-power applications would be encouraged to use higher-price (and higher profit) tubes.

Tube life should not be awful.

> in 1980 with 220V,  today we have 230-240

What is the *heater* voltage?

220 to 240 is only 9% difference, which is "nothing" to vacuum tubes.

And I am not -too- worried about "6.3" heaters running on 6.9V, because burned-out heaters are VERY rare.

But if you are closer to 7.0V or more heater voltage, ask a professional electrician for a 240V:216V step-down auto-transformer. Must supply 400 Watts output (that amp is 200 Watts audio out so may draw up to 400 Watts from the wall.) This will be a 240:24V 2 Amp transformer specially wired to step-down voltage. It should be in a metal case with the appropriate plugs and whatever fusing is required.

In the USA I would use "ACME Transformer T-1-81047". This is a 50VA core in a box with wire-space and provisions for bushings. It comes with connection data including "buck" (step-down) auto-transformer. In your use it can supply 0.5KVA which is 500VA which is 500 Watts pure resistive or 400 Watts at typical electronics power-factor.

I see the ACME sheet (snip attached) only cites UL, CSA, and NEC, so this exact part may not meet Germany's electrical regulations. But I am sure a professional electrician will know what German-market part will do the same job.

 

[Sredna]

Thanks!

The heaters sit at about 6.7 Volts, funny enough the schematic says 6.6 volts for the 6L6GCs so that shouldn't be a problem.

Ok, seems that I have to get me some "true" 6L6GCs since most cheap ones are really just relabeled weaker siblings.

Old GE, RCA and original Fender (STR387) goes for big bucks here so I thought lowering the voltages would give me access to cheaper new production tubes...


Thanks for all answers!

 

[PRR]

Modern "guitar market" tubes of good brands should stand these voltages fine.

In a school setting where I did NOT want failures, I used modern (yet low price) EL84 (originally rated at 330v) in a 417V amplifier. In the old days it made sense for a tube factory to have several grades of plate-metal. I think all good tubes today are made of high-volt plate-metal, because total production is too small to justify keeping several grades in stock.

Are you having failures with good-brand tubes? There should be little problem finding good Chinese and Russian tubes in Germany. IMHO 530V is not an extreme voltage for any 6L6GC.

BTW, on _6L6_ (1937, metal) the original 415V rating was later dropped to 360V because of slow breakdown in the *base*, not the inside of the tube. Base materials improved a LOT after that, but the rating did not go up until the GC.

 

[Gene Pink]

Ok, so Im sitting here with a Fender Studio Bass amp and I'm getting ~530V plate voltage (should be 500V) and my cheap chinese 6L6GCs have great problems with that. 

Before you do anything, how is your bias set?

If it is set on the light side, bias it up heavy. An extra 100+ mA load from six 6L6s will drag down the plate voltage some, considering the power transformer coupling loss, in series with the HV winding and the output transformer primary winding resistance.

If that doesn't do it, the buck transformer idea already suggested works well. I had to build one for my Marshal. EL-34s really don't like 513V plate, 510V screen, so I bucked down from American 120V to what Britain thinks American voltage is, 110V.  I built mine with a 120 / 10V transformer in a separate box with an outlet, because there is no way I'm gonna modify anything inside an unmolested '72 Marshall in excellent condition.

Gene

 

[shabtek]

What is the formula for series capacitance to drop ac?

 

[PRR]

> What is the formula for series capacitance to drop ac?

I put a cap in series with my duct-fan, and it spun FASTER.

There is no "simple"formula for non-simple loads.

Rectifier-cap DC supplies are very non-simple.

As a wide ballpark this job wants several hundred uFd, with *heavy* AC current. At start-up it has to be rated 240V AC. This is a LARGE motor-start capacitor. Based on  the last large motor-cap I got, this will cost several times what a 50VA auto-former costs. And I fear it will be very unstable, huge sag. More than even a guitar-amp can stand.