New build Complete: 45W, four 6V6, non-HRM

[talbany]

[ToneMerc]

These better be sturdy 6V6s JJ have maximum values at 14 Watt At this voltage a 6L6 would be a better bet or one could locate some 5992 redbank (Bendix, Tong Sol, Richardson and very expensive) tubes if your had your heart set on 6V6 tubes.

Steve

Thanks for your input. Yep, set on a quad of 6V6's with a slightly different circuit topolgy. Between four other Dumble builds I have the big tubes covered in either quads or pairs.

thanks

TM

[Zippy]

All the above plate numbers are idle voltage, at full load the plate voltages will become lower. (Take my word for this, I don't recommend attempting to measure plate voltages with an amp dimed.

Why not?

[ChrisM]

Only thing I can think of is your going to get load nasty pops!

[Zippy]

What's the rub? Clip your leads on while the amp is off or turned down.

Duh...

[Cliff Schecht]

Agree with Zippay, I probe dimed amps all of the time. If you're worried about the pop hurting the speakers then get yourself a pair of crap speakers that you don't mind sending "damaging" impulses to. I test mostly with my SRO which can handle pops from a 50W amp all day without sweating.. I would think a 12L is just as resilient.

[renshen1957]

Wev'e built quite a few of these 4X6V amps in the HRM topology running around 460 using Marshall OP Iron and let me tell you it has far more headroom than 2 6L's..We use the JJ's as well and yes they are a tough ass tube.. I have not had 1 complaint of premature failure with them in my 4X6V amps.. Burn them in..

Good Luck TM

Tony

Hi Tony,

The information I gave for Tonemerc was based on using four 6L6 tubes. His design goal was 35watts, his choice of OT would be in 50 watt+ using either 4 each 6L6 or four each 6V6.

The scuttlebutt is the JJ tube is a either a 6L6 based tube (either a hybrid design or a derated on paper 6L6).

I have yet to use the tube, or EH or any of the other tubes on the market that are supposedly to be able to higher voltages (the Old 6V6 could with stand over 1200Volts on 15% duty cycle in TV retrace circuits.

As to JJ, I have had very good experience with their KT77 as an EL34 replacement. Higher headroom (and higher plate values).

Best Regards,

Steve

[renshen1957]

Agree with Zippay, I probe dimed amps all of the time. If you're worried about the pop hurting the speakers then get yourself a pair of crap speakers that you don't mind sending "damaging" impulses to. I test mostly with my SRO which can handle pops from a 50W amp all day without sweating.. I would think a 12L is just as resilient.

Hi Cliff,

I use VTVM to probe all the time, but you hit the nail on the head as to the speaker damage. No time to elaborate earlier, day job takes precedent over posts.

Best Regards,

Steve

[talbany]

Steve
Thanks for the shout back!!...A couple things..

The scuttlebutt is the JJ tube is a either a 6L6 based tube (either a hybrid design or a derated on paper 6L6).

I would say closer to a 7591 ( Power Pentode) / 6V Hybrid..

(the Old 6V6 could with stand over 1200Volts on 15% duty cycle in TV retrace circuits.

AFAIK.. TV manufactures in the old days were very careful never to exceed the voltage ratings posted by the tube manufactures specs for obvious liability reasons.. The general purpose use for a 6V/6K's in the older TV's were used for both audio and to drive the Flyback transformers that generated the 1200V.... that drove the CRT's (retrace etc..)..The actual plate voltages on 6V's in this circuit were really around 265V..If you have a schematic of one of these 6V's running 1200v on the plates.. I would LOOOVE to see it..

As to JJ, I have had very good experience with their KT77 as an EL34 replacement. Higher headroom (and higher plate values).

Me too! KT-77 A sweeter smooth almost 6L-ish top end.. and yep extended headroom..

All The Best

Tony

[ToneMerc]

I haven't had a chance to mess with it in a couple of days, but here's the latest.

TM

[ToneMerc]

It's winding down now; outstanding items are filaments, screen & grid resistors, two remaining strands of coax and wire in the PT.

TM

[jelle]

Nice! I dig the Sylvania resistors.

[renshen1957]

Steve
Thanks for the shout back!!...A couple things..

The scuttlebutt is the JJ tube is a either a 6L6 based tube (either a hybrid design or a derated on paper 6L6).

I would say closer to a 7591 ( Power Pentode) / 6V Hybrid..

(the Old 6V6 could with stand over 1200Volts on 15% duty cycle in TV retrace circuits.

AFAIK.. TV manufactures in the old days were very careful never to exceed the voltage ratings posted by the tube manufactures specs for obvious liability reasons.. The general purpose use for a 6V/6K's in the older TV's were used for both audio and to drive the Flyback transformers that generated the 1200V.... that drove the CRT's (retrace etc..)..The actual plate voltages on 6V's in this circuit were really around 265V..If you have a schematic of one of these 6V's running 1200v on the plates.. I would LOOOVE to see it..

As to JJ, I have had very good experience with their KT77 as an EL34 replacement. Higher headroom (and higher plate values).

Me too! KT-77 A sweeter smooth almost 6L-ish top end.. and yep extended headroom..

All The Best

Tony

Hi Tony,

The information on the retrace circuits and vertical deflection voltages originated in the Design Maximums ratings and using the 6V6 as a triode in these circuits. With a 15% duty cycle the tube would be off 85% (of course you know this) the circuit may have a maximum continuous value of 350 Volts, but would receive pulses at a maximum of 1200 Volts.

GE lists this in their tube data for 6V6, 5V6, and 6V6 GTA tubes.

http://frank.pocnet.net/sheets/135/6/6V6GTA.pdf

As did Tong Sol, earlier.

Most tube data list the typical Design Center values, basically what the Design Center second guessed how the tubes would be used.

The 7591s JJ produces have a reputation of not reaching the full power potential compare to NOS Tubes or the Russian 7591A, not that it would matter that much. I have yet to audition a pair in my 4X7591 amp (the NOS tubes sound so good for the time being), although I purchased a back up set a while ago.

Best Regards,

Steve

PS, I will see if I can find any of my Dad's TV repair stuff.

[talbany]

Double

[talbany]

Steve
Looking at the 6v6gt spec sheet below..Check the 1200V peak positive plate voltage is under Vertical deflection operation only and blank for class A and no rating in AB1.. What does that tell you?

Yes, the tube is tough and can handle up to 1.2kv peak positive pulses (as stated in the data sheet) within a certain duty cycle. In the 6V6 case, 15% of one scanning cycle. That was 30 frames or 60 sweep scans per second. Read the notes at the bottom of page 2 6v6gt sheet. But the tube plate voltage rating was still only 315v. So yes, the tube has a 1200v rating, but only under certain special circumstances involving brief pulses of 2.5 milliseconds, not continuous audio work. So the 1.2kV rating doesn't apply to us at all.I wish it did...Apples and Oranges..

For audio work the normal specs of Pa=14W and Pg2=2.2W is valid, i.e. 16.6W in total: This is plate and screen dissipation (in practice we typically keep things a little below design maximum plate dissipation of 14W). I’ve seen several amps like the Deluxe Reverb hit these old tubes with 430V or so and they worked. We also know that tubes are under spec’d for the most part – 400V is most likely OK for the 6v. But, try applying 470 or 550V crank it up and see how long your nice NOS shit lasts……

for grins, I found an old TV schematic that used a 6V6 as a vertical deflection amp and have attached the schematic and voltage chart. Note that the voltages are within design max.



Tony