KT88 users - I have a question

a DIY, modified Mullard 5-20 monoblock design

Re: Another good choice.

Postby Shannon Parks » Sat Sep 01, 2007 6:28 am

Ty_Bower wrote:It made me think of this article I read about deriving the bias off the center tap of the full wave rectifier.


Just recently Eric Hayes send me a TAL 1773 amp of his to demo. It is a PP EL84 amp with fixed bias and 6GU7 LTP phase splitters. It really acts more like a baby Eiclone than a EL84-type amp - impressive! So, I was thinking of a mod for the ST35 to add a fixed bias source, and was considering the center tap method. My worry is that a resistor sized large enough for the bias will also have poor regulation, too, ude to the changing currents. I had forgotten about the zener center tap method. That seems reasonable to me.

For those interested, for an DIY35 mod I would probably go hybrid. Maybe get a fixed -12V bias and leave a much smaller cathode resistor in there.
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Postby EWBrown » Wed Sep 05, 2007 8:44 am

The "back biasing" method, is mostly good for a single channel PP or SET amp, IIRC, the WE91 used this approach, and the 30 watt amp circuit in the back of the RCA tube manual also uses "back biasing". FWIW some of the newer Bottlehead designs use this approach, in the SEX amp, the 6DN7 power section is directl grounded, and each channel's bias (it uses a dual secondary power trannie) is derived by "back-biasing" with a 620 ohm 3W WW resistor,in order to generate 27 ma plate current, and approximately -17 VDC grid bias.

Trying to back-bias two channels and 4 (or 8 power tubes) is just askin' for trouble :o Yellow_Light_Colorz_PDT_03

Typically EL84s and their ilk like to see -12 to -14 VDC at the G1 relative to cathode, fo r 35 mA idling current, though I've seen as low as 10.5VDC and high as 16VDC in some of the generic Russian "Raketa" 6P14Ps and 6P14P-EVs.

/ed B in NH
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Postby Vince » Wed Sep 05, 2007 9:16 am

[quote=" the generic Russian "Raketa" 6P14Ps and 6P14P-EVs.

/ed B in NH[/quote]

Hi Mr. Brown,
How does one determine if a 6P14P is a "Raketa"?
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Postby TerrySmith » Wed Sep 05, 2007 9:53 am

What if you used a voltage doubler on the filament supply for bias?
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Raketa = Rocket

Postby EWBrown » Wed Sep 05, 2007 12:49 pm

The marking on the side of the tube looks (sort of) like a rocket. I'll try to locate a clear photo of this marking. Many other companies also made these tubes, so YMMV, etc. These are the dirt cheap 6P14Ps, from various e-bay sellers. I've gotten batches of 16 for around $40, including the shipping, though that was a couple or more years ago... My last 6P14P order from "anthonywest" he sent me the -EB versions at no extra cost. :o Yellow_Light_Colorz_PDT_06


The generic tube boxes just read "Radio Lampa" (radio tube) in Russian Cyrillic. The old soviet system wasn't into name brand identity.

/ed b in NH
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Fixed bias EL84s

Postby EWBrown » Thu Sep 06, 2007 7:16 am

If you are using one of the AnTek toroidal 300-0-300 power trannies, it has an "extra" 12VAC winding, this can be rectified and the fixed grid bias voltage can be derived from that winding as required. The 12VACcould be voltage doubled than divided down as required.


Alternate scheme for PA774 and others, use a 24VAC trannie (the current rating can be fairly low) abd wire the 24V side across the 6.3VAC filament, then rectify, filter and resistively divide the bias voltage from the "primary" side. This allows for individual grid bias setting for each tube - use a 10 ohm resistor between cathode and ground and set to 350 mV across the resistor in order to set the idle bias at 35 mA for each.

/ed b in NH
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Re: Raketa = Rocket

Postby REALab » Mon May 13, 2013 9:54 pm

EWBrown wrote:The marking on the side of the tube looks (sort of) like a rocket. I'll try to locate a clear photo of this marking. Many other companies also made these tubes, so YMMV, etc. These are the dirt cheap 6P14Ps, from various e-bay sellers. I've gotten batches of 16 for around $40, including the shipping, though that was a couple or more years ago... My last 6P14P order from "anthonywest" he sent me the -EB versions at no extra cost. :o Yellow_Light_Colorz_PDT_06


The generic tube boxes just read "Radio Lampa" (radio tube) in Russian Cyrillic. The old soviet system wasn't into name brand identity.

/ed b in NH

Hi -
Old thread, but I thought I'd clear this up. The "Rocket" logo you're referring to is from the Voskhod factory in Kaluga, Russia.
Established in 1960 as the "Калужский Завод Трубки Злектрон" (Kaluga Electron Tube Factory), they became JSC Voskhod (Russian for "sunshine") in 1993, and changed their logo
from: Image

to: Image

They still manufacture 6Н1П (6H1P), 6Н1П-ЕВ* (6H1P-EV), 6Н2П (6H2P), 6Н2П-ЕВ* (6H2P-EV), 6Н2П-ЕВ* ОС (6H2P-EV OS), 6EU7, 6Н23П (6H23P), 6Н24П (6H24P), 6Ж1П (6F1P), 6Ж1П-ЕВ* (6F1P-EV), and 6Ж1П-ЕВ* ОС (6F1P-EV OS) tubes, as well as ICs, transistors, photodiodes, and laser components.

* For those that don't know, the "-ЕВ" ("EV" in Roman letters) is added to indicate ("E") extended service, and ("B") special selection for low noise/microphonics.
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