parallel tube rectifiers?

[bigugly]

Considering the fact that I'm a cheap ass, I was trying to slap together a low watt gtr amp. I'm using rescued parts from two old tube intercoms that I picked up at Apex.

The parts from this are: two of each

Pwr Xfmr: Pri is 115, Sec are 290-0-290 winding, 16 winding, 6.3 heater, and a single wire that is 15v when referenced to the CT of the main secondary.

Rectifier: 6X4

Pre Amp valve: 12AX7

Power valve: 6CM6

I'm basing my amp off of this schematic: http://annex.ax84.com/media/ax84_m161.pdf

I would likw to use an EL84 in the output stage and I also have some 6135 single triodes that I'd like to play around with in this amp. Now after looking at the data sheets fro all these tubes I've come to the conclusion that in the worst case scenario, ie. EL84 otuput with four 6135s instead of two 12AX7s, the 6X4 will exceed it's max current output.

So here is the question. Can I run two 6X4s in parallel and combine their output before the first filter cap? Or, possibly, feed both rectifiers from the secondary and have one for the power tube and the other for the pre amp tubes?

Like I said I'm cheap and I don't want to spring for a better rectifier tube like an EZ81.

Thanks in advance,

James

 

[kiira]

[quote author="bigugly"]So here is the question. Can I run two 6X4s in parallel and combine their output before the first filter cap? Or, possibly, feed both rectifiers from the secondary and have one for the power tube and the other for the pre amp tubes?

Like I said I'm cheap and I don't want to spring for a better rectifier tube like an EZ81.

Thanks in advance,

James[/quote]

Hi James... yeah you can parallel rectifier tubes it's done all the time. I've bulit amps with paralleled ez81s and 6by5s. But trust me, I've built the Hi Octane and first used used an ez81 but changed it out for some 4007s. So yah spring for the 14 cents because the amp sounds better with SS rectification. It's single ended - no PS sag anyway if that is what you like.

Kiira

 

[bigugly]

Thanks Kiira,

Good to know about the SS rectifiers in the HO. I guess I'll save the 6X4s for a PP amp.

James

 

[PRR]

> Can I run two 6X4s in parallel and combine their output before the first filter cap?

Yes. Just do it.

Your B+ may wind up higher than shown on the plan. You may have to reduce power-tube current to stay inside safe plate dissipation. You may not really be past a 6X4's rating.

If you go solid-state, a 290-0-290 winding will give nearly 400VDC, not the 250V shown in that plan. That's past a EL84's book-rating. Even though good EL84 will take 350V and more, it will have to be biased to lower current so it won't melt from the heat. Then for best power, the OPT should be up near 8K-10K.

> Power valve: 6CM6 ...I would likw to use an EL84 in the output stage

If you have the 6CM6.... it looks to me like an EL84 (or maybe 6V6) re-rated (and maybe re-pinned) for TV sweep duty.

> I also have some 6135 single triodes that ... come to the conclusion that ... with four 6135s instead of two 12AX7s, the 6X4 will exceed it's max current output.

Unlikely. The 6135 datasheet does show 10mA typical, for VHF work, but you would NOT run it that way for guitar-amp voltage gain stages. That suggests plate resistors of 10K or less; the tube will give ample gain at the 100K level typical of voltage amps.

Frequency response will not be MHz at ~1mA, but how high can you fret a gitar? Response should extend well past 20KHz, but your tone-stack and output iron will limit that to 7KHz-15KHz, and your speaker won't go past 5KHz (nor would you want that).

 

[PRR]

6CM6 is a mini-base 6V6 rated for TV duty. It will make about as much power as EL84, but the cathode bias resistor needs to be twice as big.

> two old tube intercoms

Then someone already did the hard figuring. The power output is about all you can get on that power transformer and rectifier; commercial designers don't leave much on the table. The difference between an intercom and a guitar amp is mostly excess switching, adding tone-shaping controls, and an output transformer that will pass below 200Hz. Oh, a few coupling caps to pass more balls, and maybe more filtering to lower hum, but you'll probably be replacing most caps anyway, just supersize them.

 

[bigugly]

Informative as always, PRR. I do have another power trnsformer that is 200-0-200.

I set up the schematic, as per the linked schem, with the smaller PT in PSU Designer II from Duncan Amps and I get 227v with a load of 5k on the supply.

You are correct, Prr. I was using the max current per tube to calculate my PT and rectifier needs. I can see how that would mislead me.

I'll put the 6X4 into the amp since there is a hole already there for it.

Thanks again all for the insight.

James