Discussion:
Answer to an FAQ: Using 6V6's In a Dynaco ST-70
(too old to reply)
Casino
2003-08-15 07:18:07 UTC
Permalink
Hi All,

In several of my previous postings, I have mentioned the use of
6V6GT's in my Dynakit ST-70, and I'm often asked how is it possible,
etc.

So, for those of you who are wondering, my ST-70 is stock
(unmodified). So inorder to use 6V6's safely, the following changes
must be made:

Replace the 5AR4 with 5U4G or 5R4GYB.
Reduce bias to half of EL34 bias (0.78 volts on on Biaset reading for
6V6).
Connect 8-ohm speakers to 4-ohm taps (suggestion from Ned Carlson).

So how does it sound? With RCA black-plates (6V6G's and 6V6GT's) or
Electro-Harmonix 6V6EH's, it sounds better than with any 6CA7 or EL34
types. I'm saving the RCA's for future radio restoration projects so
I'll be listening to the EH's instead. One "expert-listener"
described the sound of 6V6's in my ST-70 (BOTH the RCA and EH) as
"liquid", "warm" and "smooth" (and I have absolutely NO IDEA what
those words mean). For comparison, I've also tried old RCA 6L6G's and
6L6GB's in my ST-70; they were described as "dry" sounding. (I'm also
saving those 6L6G's for radio restoration projects so I won't be using
them in my ST-70 again, except for brief demos.

If power is not an issue, I highly recommend 6V6EH's (from
Electro-Harmonix) if you're using a Dynaco ST-70. After several hours
of burn-in, they (EH's) are indistinguishable from RCA black-plates.
Please save those vintage RCA's for old radios in which they belong.

Regards,

C.W.
William Sommerwerck
2003-08-15 10:53:27 UTC
Permalink
One "expert-listener" described the sound of 6V6's in my ST-70
(BOTH the RCA and EH) as "liquid", "warm" and "smooth"
(and I have absolutely NO IDEA what those words mean).
Liquid or smooth is the opposite of grainy. Think of a photo with visible grain
and one with no grain at all. As a reviewer once put it, grainy is as if the
sound image is made of tiny particles. Transistor equipment was often accused of
being grainy-sounding, something you never heard from tubes.

Warm (as opposed to cool) generally means a bit of boost in the midbass. Tube
equipment (even, oddly, devices without transformers) tends to sound warm.
Transistor equipment leans toward neutral or cool.

The sound of your modified S-70 is probably an improvement, as an unmodified
ST-70, like many tube power amps of that era, tends to be hard and bright.
Shiva
2003-08-15 14:55:53 UTC
Permalink
Post by William Sommerwerck
One "expert-listener" described the sound of 6V6's in my ST-70
(BOTH the RCA and EH) as "liquid", "warm" and "smooth"
(and I have absolutely NO IDEA what those words mean).
Liquid or smooth is the opposite of grainy. Think of a photo with visible grain
and one with no grain at all. As a reviewer once put it, grainy is as if the
sound image is made of tiny particles. Transistor equipment was often accused of
being grainy-sounding, something you never heard from tubes.
Warm (as opposed to cool) generally means a bit of boost in the midbass. Tube
equipment (even, oddly, devices without transformers) tends to sound warm.
Transistor equipment leans toward neutral or cool.
The sound of your modified S-70 is probably an improvement, as an unmodified
ST-70, like many tube power amps of that era, tends to be hard and bright.
My spectrum analyzer has the following features:
1. Warmth indicator (calibrated in Bil.H.'s)
2. Liquidity indicator (calibrated in cm^3 through 1mm diameter jet @ 1PSI)
3. Smoothness meter (calibrated in m^-1000)
4. Graininess meter (calibrated in audiophools).
5 Cristalinity meter (in Mols)

I don't know what I'd do without these features...
John Stone
2003-08-15 12:49:56 UTC
Permalink
Post by Casino
Hi All,
In several of my previous postings, I have mentioned the use of
6V6GT's in my Dynakit ST-70, and I'm often asked how is it possible,
etc.
So, for those of you who are wondering, my ST-70 is stock
(unmodified). So inorder to use 6V6's safely, the following changes
Replace the 5AR4 with 5U4G or 5R4GYB.
You do realize that the 5U4 has a heater current of 3 amps vs 1.9 for the
5AR4. What exactly does this substitute accomplish?
Post by Casino
Reduce bias to half of EL34 bias (0.78 volts on on Biaset reading for
6V6).
Connect 8-ohm speakers to 4-ohm taps (suggestion from Ned Carlson).
You are not reducing the bias, you are increasing it. When you increase
bias, you reduce current. All other things being equal, you are cutting off
each half of the output pair sooner, introducing the possibility of
crossover distortion.
Post by Casino
So how does it sound? With RCA black-plates (6V6G's and 6V6GT's) or
Electro-Harmonix 6V6EH's, it sounds better than with any 6CA7 or EL34
types. I'm saving the RCA's for future radio restoration projects so
I'll be listening to the EH's instead. One "expert-listener"
described the sound of 6V6's in my ST-70 (BOTH the RCA and EH) as
"liquid", "warm" and "smooth" (and I have absolutely NO IDEA what
those words mean). For comparison, I've also tried old RCA 6L6G's and
6L6GB's in my ST-70; they were described as "dry" sounding. (I'm also
saving those 6L6G's for radio restoration projects so I won't be using
them in my ST-70 again, except for brief demos.
Have you tried a power or distortion measurement on your ST70 with these
tubes? What do you want to bet both measurements are seriously compromised.
That "lush" sound you are hearing is probably a bunch of second order
harmonic.
Post by Casino
If power is not an issue, I highly recommend 6V6EH's (from
Electro-Harmonix) if you're using a Dynaco ST-70. After several hours
of burn-in, they (EH's) are indistinguishable from RCA black-plates.
Please save those vintage RCA's for old radios in which they belong.
Sorry, but I think the whole exercise is ridiculous. You basically have the
wrong tubes in the amplifier. 6V6's have much lower plate dissipation than
EL34's. There's an impedance mismatch, and the plate voltage is probably too
high. The functional equivalent of a 6V6 is the lowly 6AQ5: a tiny 7 pin
tube used in cheap hi fi systems in the 50's and 60's. 6v6's were available
when the Stereo 70 was designed. If they work so well, why didn't Dynaco
recommend them?
George R. Gonzalez
2003-08-15 13:43:02 UTC
Permalink
Post by Casino
Hi All,
In several of my previous postings, I have mentioned the use of
6V6GT's in my Dynakit ST-70, and I'm often asked how is it possible,
etc.
I'd say it's possible, in the sense that both tubes have 8-pin sockets.

Now whether it's a *good* idea, and will work well, for more than 15
minutes,
is I suspect an open question. Typically your 6CA7's are run at plate and
screen
voltages that are waaaaay above 6V6's maximums.

Also your 6V6 has much lower plate dissipation than a 6CA7, about half.

And oh, the transconductance of a 6V6 is only about one THIRD that of the
6CA7,
so the overall gain and power out could be as little as ONE NINTH as much as
originally.

Moving the load to the 4-ohm taps has its pluses and minuses. On the plus
side,
you maybe able to eke out a little bit more power. On the minus side,
you'll be
doubling the transformers working impedance, which will roll off the low
frequency response at about twice it's normal place.

Overall it doesnt sound like the best modification one coudl make.

Full disclosure: In an attempt at false economy, I've replaced expensive
7868's with cheeep 6GT5's. Not a superb idea, but it kinda "works". You do
have to reduce the screen voltage and increase the bias.
But the tubes do fit in the sockets, and you do get some pretty good
sounding power out, but it's only about half the normal amounts. And the
tubes are being run within their ratings. It's a amtter of opinion whether
this should be considered an admirable way to get old aps up and running
again, or it's a desecration of the way things were meant to be.
Patrick Turner
2003-08-15 15:47:31 UTC
Permalink
Post by George R. Gonzalez
Post by Casino
Hi All,
In several of my previous postings, I have mentioned the use of
6V6GT's in my Dynakit ST-70, and I'm often asked how is it possible,
etc.
I'd say it's possible, in the sense that both tubes have 8-pin sockets.
Now whether it's a *good* idea, and will work well, for more than 15
minutes,
is I suspect an open question. Typically your 6CA7's are run at plate and
screen
voltages that are waaaaay above 6V6's maximums.
I'd tend to agree that if one just plugged 6V6 straight in,
and cranked the volume up, maybe you'd see red hot plates pretty soon.

ST70 is a UL amp, so the high screen and plate voltage
would mean idle disspation would be above ratings.
perhaps placing a series R in the PS to lower B+ down to
about 380v might help.
Then adjust the Rk, and bias the tubes for about 10 watts of Pd at idle.

Then always connect the 8 ohm speakers to the 4 ohm taps, to raise the load a-a
to suit the 6V6.
Expect about 17 watts max at clipping.
Post by George R. Gonzalez
Also your 6V6 has much lower plate dissipation than a 6CA7, about half.
see above.
Post by George R. Gonzalez
And oh, the transconductance of a 6V6 is only about one THIRD that of the
6CA7,
Doesn't matter much.
Post by George R. Gonzalez
so the overall gain and power out could be as little as ONE NINTH as much as
originally.
Nah, 2 x 6V6 can give up to 20 watts with reliablity.

BUT, if they were connected in tetrode mode, with Eg2 at
say 250v, then Ea could be 450v, no wories.
Then with the higher RLa-a, you get some power.
Post by George R. Gonzalez
Moving the load to the 4-ohm taps has its pluses and minuses. On the plus
side,
you maybe able to eke out a little bit more power.
Indeed.
Post by George R. Gonzalez
On the minus side,
you'll be
doubling the transformers working impedance, which will roll off the low
frequency response at about twice it's normal place.
Nah, ST70 has ample inductance in its OPTs, and the NFB will
keep the response level.
Post by George R. Gonzalez
Overall it doesnt sound like the best modification one coudl make.
Well no, but you can't stop some ppl.

If they only want 3 watts, no problem.
Post by George R. Gonzalez
Full disclosure: In an attempt at false economy, I've replaced expensive
7868's with cheeep 6GT5's. Not a superb idea, but it kinda "works". You do
have to reduce the screen voltage and increase the bias.
But the tubes do fit in the sockets, and you do get some pretty good
sounding power out, but it's only about half the normal amounts. And the
tubes are being run within their ratings. It's a amtter of opinion whether
this should be considered an admirable way to get old aps up and running
again, or it's a desecration of the way things were meant to be.
I once converted a Lux amp with 7868 to EL34 operation.
I had to make the metal box taller, and replace the tube sockets,
but it worked OK.

Patrick Turner.
Casino
2003-08-16 04:54:32 UTC
Permalink
Best EL34: Svetlana
Anyone have an opinion on the best EL34's for an ST-70? I've got two of
them.
George R. Gonzalez
2003-08-16 14:12:39 UTC
Permalink
Post by Patrick Turner
ST70 is a UL amp, so the high screen and plate voltage
would mean idle disspation would be above ratings.
perhaps placing a series R in the PS to lower B+ down to
about 380v might help.
???? It's generally a very very very poor idea to use a resistor to drop the
voltage to a Class AB
amplifier!!!

Firt of all, that's going to result in massive droop under large-signal
conditions.
This setup is already going to cut the output power by a big fraction, you
can't afford to sacrifice muc more by introducing resistors!
Post by Patrick Turner
Post by George R. Gonzalez
And oh, the transconductance of a 6V6 is only about one THIRD that of the
6CA7,
Doesn't matter much.
It wont matter at ALL if the driver tubes can put out three times as much
grid-swing, and that wide swing is within the 6V6's saturation
to cutoff range.. Can you verify this?
Post by Patrick Turner
Post by George R. Gonzalez
Moving the load to the 4-ohm taps has its pluses and minuses. On the plus
side,
you maybe able to eke out a little bit more power.
Indeed.
Post by George R. Gonzalez
On the minus side,
you'll be
doubling the transformers working impedance, which will roll off the low
frequency response at about twice it's normal place.
Nah, ST70 has ample inductance in its OPTs, and the NFB will
keep the response level.
Seems unlikely Dynaco would have supplied twice the low-end
inductance than what's needed. Anything less than that you're
going to encroach on with the tap-switching.
Post by Patrick Turner
If they only want 3 watts, no problem.
Good point, some folks like to listen at less than earshattering levels,
or they have efficient speakers. Those of us with tiny speaker budgets
will have to cope somehow.

Regards,

George
Patrick Turner
2003-08-17 03:01:28 UTC
Permalink
Post by George R. Gonzalez
Post by Patrick Turner
ST70 is a UL amp, so the high screen and plate voltage
would mean idle disspation would be above ratings.
perhaps placing a series R in the PS to lower B+ down to
about 380v might help.
???? It's generally a very very very poor idea to use a resistor to drop the
voltage to a Class AB
amplifier!!!
But the supply voltage in an ST70 appears to be too high for
6V6 UL operation, where the applied grid bias would have to be
much higher to make the idle current about half what it is with EL34,
to achieve a Pd which is lower, ie, about 10 watts max.
Reliability comes into the picture

The amount of class A with 6V6 would be a lot lower,
and thd will be a lot higher for the amp.
Post by George R. Gonzalez
Firt of all, that's going to result in massive droop under large-signal
conditions.
Indeed, but with music signals, and if you used a 470 uF
B+ supply cap, the undulations in B+ would be minimal.

If max power is important, then try the full B+.
If you want a maximum class A power, try with a lower B+, and
higher idle current.
Post by George R. Gonzalez
This setup is already going to cut the output power by a big fraction, you
can't afford to sacrifice muc more by introducing resistors!
It depends on what is wanted as a final outcome.

Tetrode operation with a lower Eg2 supply would allow
a high B+, and then the maximum power could be extracted,
but more NFB may have to be applied to keep the thd low,
and the Rout low, to compensate for the absense of screen FB
from the UL connection.
Post by George R. Gonzalez
Post by Patrick Turner
Post by George R. Gonzalez
And oh, the transconductance of a 6V6 is only about one THIRD that of
the
Post by Patrick Turner
Post by George R. Gonzalez
6CA7,
Doesn't matter much.
It wont matter at ALL if the driver tubes can put out three times as much
grid-swing, and that wide swing is within the 6V6's saturation
to cutoff range.. Can you verify this?
6V6 require less voltage drive compared to EL34,
under similar conditions. Less maximum output power
is generated, so I'd say the drive voltages for 6V6 would always be
able to be produced by the ST70 driver arrangement,
which is a concertina phase inverter driver, and pentode input
tube, which has its anode load tied to the cathode load of the concertina,
thus providing a bootstrapped load for the pentode, to dramatically
boost the pentode gain.
Post by George R. Gonzalez
Post by Patrick Turner
Post by George R. Gonzalez
Moving the load to the 4-ohm taps has its pluses and minuses. On the
plus
Post by Patrick Turner
Post by George R. Gonzalez
side,
you maybe able to eke out a little bit more power.
Indeed.
Post by George R. Gonzalez
On the minus side,
you'll be
doubling the transformers working impedance, which will roll off the low
frequency response at about twice it's normal place.
Nah, ST70 has ample inductance in its OPTs, and the NFB will
keep the response level.
Seems unlikely Dynaco would have supplied twice the low-end
inductance than what's needed.
There is enough inductance.

Sure, if you ran the amp in pentode mode, and without NFB,
with an 8 ohm load on the 4 ohm tap, then the LF pole
would be a lot higher than what it is with EL34 in UL,
since the effective Ra-a, and RLa-a which is in parallel
with the Lp inductance is higher.
20 dB of NFB is plenty to flatten the response, regardless of
what is used, UL, or tetrode operation.

The only thing that worries me with terode operation would be the
stability of the amp; the ST70 OPTs are not the best OPT
that's ever been made.

Whatever is done, one has to be careful about stability issues,
and having zobel networks across each half primary of the OPT
would be a good idea, to make sure there is a load applied to the
tubes at above 50 kHz, to limit gain at HF, and improove HF stability with NFB.


Anything less than that you're
Post by George R. Gonzalez
going to encroach on with the tap-switching.
Post by Patrick Turner
If they only want 3 watts, no problem.
Good point, some folks like to listen at less than earshattering levels,
or they have efficient speakers. Those of us with tiny speaker budgets
will have to cope somehow.
Regards,
George
Well nobody can have it all.

Ear bashing level, lower price 6V6,
and insensitive speakers just don't go together.
A compromise is involved.
Many modern speakers need a watt to make 89 dB SPL,
but they can take higher power applied, and have a flatter response.
Most modern amps are solid state, and at least 50 watts.
In 1965, many speakers made 95 dB with the same watt of power,
and these need only 1/4 watt to make 89 dB.
12 watt amps were regarded as adequate.
The speaker power handling was poorer, and the response wasn't as flat,
because the lighter cone materials using mainly paper/cardboard were
more prone to resonances and cone break.
Depending what you bought, they varied from good
to downright godawful.

So if one decides to use 6V6 and settle for maybe 20 watts,
instead of 35, then just how you use the amp with what speakers
begins to get more critical.

I once rewired a UL amp with 2 x EL34, that a client had inherited from
his dad. I had speakers rated at 93 dB, and he was quite unimpressed
by the volume level able to be achieved, although I found
it far too loud for normal listening, as would everyone else I know.
He needed maybe 120 watts from 4 x 6550,
but he was of course, cashless.

I get by happily with 89 dB speakers, and 22 watt class A SE amps.

Patrick Turner.

Fred Nachbaur
2003-08-16 14:41:50 UTC
Permalink
Anyone have an opinion on the best EL34's for an ST-70? I've got two of
them.
Zoid
z9design.com
Ones with an intact filament, good emission, reasonably high
transconductance, no gas, and low grid emission.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+
Chenchen Ku
2003-08-16 07:20:44 UTC
Permalink
I try 6V6 too a while ago, I also use 5U4G too to lower the B+, I actually
use a worn out 5U4G so it will drop more voltage than a regular 5U4G.
Yes, 6V6 sound quite sweet and smooth, just a little under power.

For ST70 fans on a budget, I highly
recommand Mullard EL38, you'll get about the same watts output as EL34,
and same quality from Mullard, but much cheaper price ($10-$20) each.
I actually prefer Mullard EL38 than Mullard EL34 these days, I recently
complete a parallel push pull amp using 4 EL38 per channel, sounds
really nice (6J7, KT66 driver, Tango NC14 interstage, UTC LS58 output)

Chenchen
Post by Casino
Hi All,
In several of my previous postings, I have mentioned the use of
6V6GT's in my Dynakit ST-70, and I'm often asked how is it possible,
etc.
So, for those of you who are wondering, my ST-70 is stock
(unmodified). So inorder to use 6V6's safely, the following changes
Replace the 5AR4 with 5U4G or 5R4GYB.
Reduce bias to half of EL34 bias (0.78 volts on on Biaset reading for
6V6).
Connect 8-ohm speakers to 4-ohm taps (suggestion from Ned Carlson).
So how does it sound? With RCA black-plates (6V6G's and 6V6GT's) or
Electro-Harmonix 6V6EH's, it sounds better than with any 6CA7 or EL34
types. I'm saving the RCA's for future radio restoration projects so
I'll be listening to the EH's instead. One "expert-listener"
described the sound of 6V6's in my ST-70 (BOTH the RCA and EH) as
"liquid", "warm" and "smooth" (and I have absolutely NO IDEA what
those words mean). For comparison, I've also tried old RCA 6L6G's and
6L6GB's in my ST-70; they were described as "dry" sounding. (I'm also
saving those 6L6G's for radio restoration projects so I won't be using
them in my ST-70 again, except for brief demos.
If power is not an issue, I highly recommend 6V6EH's (from
Electro-Harmonix) if you're using a Dynaco ST-70. After several hours
of burn-in, they (EH's) are indistinguishable from RCA black-plates.
Please save those vintage RCA's for old radios in which they belong.
Regards,
C.W.
william_b_noble
2003-08-16 16:25:27 UTC
Permalink
without going into any questions about the audio quality, why the heck to
you feel you have to change the rectifier tube?
Post by Casino
Hi All,
In several of my previous postings, I have mentioned the use of
6V6GT's in my Dynakit ST-70, and I'm often asked how is it possible,
etc.
So, for those of you who are wondering, my ST-70 is stock
(unmodified). So inorder to use 6V6's safely, the following changes
Replace the 5AR4 with 5U4G or 5R4GYB.
Reduce bias to half of EL34 bias (0.78 volts on on Biaset reading for
6V6).
Connect 8-ohm speakers to 4-ohm taps (suggestion from Ned Carlson).
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