ST-70 6B4G

I have a pair of stock ST-70's that have been recapped and sound great.  I also have a pair of MK III's.  I am having a pair of AR3a's restored and at some point hope to use all 4 Dynaco's to bi-amp using an AR EC-2 crossover, with the ST-70s in mono.  In the meantime I am going to listen to one ST-70 in Triode mode, as soon as I receive some new resistors, after that I would like to set it up to use a matched quad of Sylvania JAN 6B4G Triode tubes.  After that I will return it to stock form for use with the other ST-70.  I know how to set it up for Triode mode with the El-34s.  I am not so certain about the 6B4G's but so far I plan the following steps.

Replace the .02 Ceramic in the bias supply with 2 800 ohm sand resistors.

Move the wires connected to the bias pots to ground.

Remove the wires from points 11 and 14

Use a .22 cap instead of the .1 cap on the board.

Are there other steps I need to take, should the winding's be removed from the 4 pins.  Do I need to change the 7199's, could an adapter be used.  Any advice will be appreciated.  Thank you, ron

I'm certainly not an expert at this, but a quick glance at 6b4g spec sheet shows 325 volts max on the plate. The ST-70 is well over 400 volts.
You're going to have to do quite a bit more than change a few parts; the 6b4g is also a triode, not a pentode that the amp was designed for.
Short answer: the ST-70 was not designed for this tube in any way; don't do it!

De gustibus et coloribus non est disputandum.

Blitzen wrote:I'm certainly not an expert at this, but a quick glance at 6b4g spec sheet shows 325 volts max on the plate. The ST-70 is well over 400 volts.
You're going to have to do quite a bit more than change a few parts; the 6b4g is also a triode, not a pentode that the amp was designed for.
Short answer: the ST-70 was not designed for this tube in any way; don't do it!

The Sovtek example is rated at 450 V on the plate, max.

https://www.newsensor.com/pdf/sovtek/6b4g-sovtek.pdf  

That does not make it a good idea. At all.

I have a pair of stock ST-70's that have been recapped and sound great.  I also have a pair of MK III's.  I am having a pair of AR3a's restored and at some point hope to use all 4 Dynaco's to bi-amp using an AR EC-2 crossover, with the ST-70s in mono.  In the meantime I am going to listen to one ST-70 in Triode mode, as soon as I receive some new resistors, after that I would like to set it up to use a matched quad of Sylvania JAN 6B4G Triode tubes.  After that I will return it to stock form for use with the other ST-70.  I know how to set it up for Triode mode with the El-34s.  I am not so certain about the 6B4G's but so far I plan the following steps.

Looking over the tube data - you will get +/- 6 watts in PP. That, on the best day ever, will starve (either section or both) the 3as, no matter how well restored they are. I am feeding my pair from a 60-WPC rms, in a room that is 17 x 12 x 10, and it barely does the job for the complex passages. 6 watts would be clipping 50% or more of the time. If you are attempting to see what you *can* do, that is one thing. But this particular combination is, on the face of it, perhaps not the best use of your time and resources.

In the realm of tube amps, the entire concept of headroom is rarely discussed. And the peculiar (in the sense of unusually specific) example of the AR3a (a 4-ohm speaker) and the ST 70, this is even more rare. Those from the solid-state section of the hobby usually carry the perception that a 4-ohm speaker 'sees' twice the power from the amp than an 8-ohm speaker. Not true of tube amps, being constant-current devices. The watts-out will remain the same at any speaker impedance.

The AR 3a is rated at 86 dB @ 1 watt at 1 meter.

https://boomspeaker.com/noise-level-chart-db-level-chart/  

Chamber music in a regular auditorium (not reinforced) is 90 dB. Your 6 watts will hardly make even this from the ARs. Much less handle a highly dynamic source. Yes, you are bi-amping, but the brute fact of the matter is that these speakers are power-pigs of the first order. I also seem to remember that the Audio Research EC2 is a 2-way device. That will require some finagling to use on a 3-way speaker. I base this on the assumption that you are either removing or entirely bypassing the on-board passive crossover.  I suppose you could adjust the crossover points so that all the lows go to the woofer, and everything else goes to the tweet and mid. Do you plan to castrate the MK-IIIs similarly to the 70? I am guessing that running the 70 in Triode Mode allows for the 5-pin configuration of the 6B4?

On the 7199s, if you go with an after-market driver board, it may reduce the gain overall. The 7199 is a pentode/triode chosen for the purpose, and not only because David Hafler squeezed every nickel until Jefferson howled, but because it is a particularly good tube for the use, and so-chosen by a goodly number of other makers not so parsimonious as Hafler. There are similar tubes that may be used with an adapter, or you may rewire the socket - your choice. The 6GH8 comes immediately to mind.

In any case, have fun, and please report back on your findings and discoveries! Most of the above is simply rambling on the concepts expressed.

As the OP said, he wants to use a set of Sylvania 6B4G tubes. Here's the data sheet:
https://frank.pocnet.net/sheets/121/6/6A3.pdf
(6B4G will refer you back to this 6A3 sheet; its base is different otherwise the same)
The new-production 6B4G by New Sensor is likely not the same in other respects; they like to use parts from other more common tubes to make an "acceptable" substitute for the less-common.

Helpful.

Thanks!

I only want to use the St-70 in EL-34 triode mode for comparison, to pentode mode.  My resistors came today so I can do that.  I will use Marantz Imperial 6's and EV-4's, for the Triode mode, they are both relatively sensitive.  For the AR3a's I intend to return the St-70 to stock form and use both of them set in mono(I have 2) for the high and mid connector, on the AR's and the MkIII's on the woofers.  The EC-2 is set at 1000hz, I will not disturb the internal crossover.  I think this is called vertical bi-amping. I also have an EC-4 that is set for 2 way and it is at 100 hz, I will try that also at some point.

About the 6B4G's.  I thought I remember reading something in Glass Audio or a similar publication about the ST-70 and 6B4G's.  I am beginning to think I may be wrong about, that and am reconsidering using them.

Thanks again for the help.

For a couple of days I have listened in Triode mode for the first time.  Right now an Iphone is feeding an M4A signal to an Audioengine1 bluetooth streamer hooked to an Ampex 403 preamp, going to an AR EC-4 crossover set for 2 amps.  100hz and below go to a Pilot SA-260 hooked to EV-4 speakers, the rest goes to the ST-70 feeding Marantz Imperial 6 speakers.  The sound is wonderful.  20 years ago I recapped using .22's where the .1's were, I may change back to the .1s using Russian oils.  The B+ is a 47mf Cerifine followed by 3 20mf Solens, it is silent.  The Bias caps are 100/100 Sprague Atoms.  I have several 7199's and see no reason at this time to change the board.  I have listened with just the St-70, and was very pleased, of course playing around with a bunch of stuff is part of the fun.