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I'm using a VTA ST-70 and wanted to ask a question about the two outer phase gain tubes. I understand that the center tube has the most effect on sound, while the outer tubes add very little or have no effect.
I'm not finding that?? Yes, there were minor (if any?) differences while using different brands of 12AU7's. However, I popped in a set of 5963's and heard quite a difference in sound. Didn't care for it, but it was noticeably different.
I prefer the 12BH7 in the center position. Would it be worth it to use them in the outer positions as well?
6SN7GTB using socket adapter is another good option.
I'm assuming that would convert the 12AU7 board into the 6SN7 board that is offered as a drop in replacement. However, even Bob didn't feel it did anything more or less than the original 12AU7 version.
The problem is the non-linearity of the 12au7. Previously I changed out 12au7's with Tungsram E80CC. It wasn't until recently I found more improvement with 6SN7's.
Maybe it is something with your application? You could look at where you are on the curves. Does it just operate in the linear region?
The sound improved (not subtly either) by changing the 12Au7 out of the center position and putting in a more linear 12BH7. The only reason I haven't changed the outer two positions is because Bob said it made no difference. Only until recently have "I" noticed - it DOES.
I was never a big fan of the 12AU7. WHY I bought them, I don't know???
Honestly, if this doesn't improve the sound while using the EL34 output tubes, I'm selling the amp. I bought this amp to use those tubes. It sounds WONDERFUL with the KT66's, but unfortunately I didn't want a KT66 type amp.
My experience with KT66's was with Quad II amps. I found some original Genelex KT66's and wasn't impressed. I tried some 6l6gc black plates and the difference was significant.
If you do replace your amp, what are you thinking of buying?
I am MORE than happy using my Sherwood S-5000 (EL84M Russian tubes).
I ran a dedicated input line directly into the driver tube and bypassed the entire preamp section. The 7199 driver is now a Amperex 7643 gold pin driver tube. The entire unit has been fully restored/upgraded by Mike Samra.
There is nothing like the sound of a good EL84. It's amazing and I always loved it.
My summer time amp is a fully restored Marantz 250M. To me it just ain't getting much better than these two amps.
So what am I getting if I sell the VTA - who knows???? But I know I don't need or really want anything else right now.
I agree with you about EL84 amps. I have some Heathkit UA2's I really like and a couple of EL84 integrated amps. Unfortunately my speakers do better with more power. What speakers do you like with the EL84's?
Believe it or not, I'm using 85db Zaph Audio SR71's with NO problems.
I got the EH 12BH7 tubes yesterday and tried them in the splitter positions. I really don't understand how ANYONE can say, they don't make an impact. I popped them in and HOLY COW. The upper bass/lower midrange was solid as a rock! Great tone. I'm only hoping that the upper treble relaxes a little more as the tubes break in. But in short they are much better balanced than the 12AU7's and offer more "feeling" to the sound.
I'll start a new post with the results.
Thanks for asking
The EH 12BH7 tubes will settle. If I recall correctly Bob Latino stated that they handle voltage swings far better than the 12AU7. My ST-70 is currently using an NOS Tung Sol 12AU7 in the center, the two EH 12BH7 on the outboard and four Psvane EL34s for power. A Sovtek rectifier as well. It's a sweet sounding amp that is the heart and soul and as it plays tunes as well as being used for the TV it is on most of the time and is very stable.
6SN7GTB using socket adapter is another good option.
For me the magic came with the 12AU7 in the centre and 12BH7s on the outer positions. For the former I used an old stock US brand and the latter the Electro Harmonix which I have found repeatedly to be a great sounding 12BH7.
Note that this driver board uses more current from the HV, (I think around 26mA) so will push the transformer and rectifier tube to the limits. The board supplier has suggested that the output tubes be run at 40mA vs the stock 50ma to extend their life but really I think it is to offset the extra loading from the board. I compared the sound of 40mA to 50mA and found the 50mA to be significantly better. So there becomes the issue - best sound equals overloaded transformer and rectifier.
My solution was to put the amp in a wooden chassis box with an adjacent bay which houses two 5AR4s, a bigger choke, additional capacitance and install a reissue power transformer which has slightly more capacity and is better matched to today's higher line voltage. The result is a superb sounding amp and power transformer that runs slightly hot (63C).
I found the same exact thing. I ran the El34's at 40ma and they sound limp. Upped to 50ma and they have a little more life in them.
I did the diode mod, so let's see if that can hold it up. Honestly if this doesn't work out then I'm selling this amp. I've been noodling around with stereos for 40+ year now, and I done tweaking, rolling and DIYing at this point in my life.
The 12AU7 is non-linear and (IMO) is OK only in the cathode follower or "concertina" phase splitter roles. Definitely use 12BH7s in positions where a gain in signal voltage occurs.
Don't lose sight of the fact that 12BH7s draw 2X the heater current 12AU7s draw. Exceeding the capabilities of the power transformer is BAD practice.
I'm told that the VTA board can handle the draw of three 12BH7's. I'm OBVIOUSLY not an expert, but I believe the center tube splits but adds very little actual gain. I believe a fair part of the gain does occur in those tubes.
V1 is a differential gain block phase splitter, AKA long tailed pair (LTP). The theoretical max. gain for such a setup is 1/2 the amplification factor (μ). In practice, roughly 1/4 μ is obtained. Kudos to the designer for using a constant current sink (CCS) load in the LTP's "tail".
That 1/2 V2 is a ubiquitous common cathode voltage amplifier. The bottom line is that all the small signal tubes in this setup are amplifying and, therefore, the non-linear 12AU7 should be eschewed.
As long as sufficient heater current is available from the power transformer, only 12BH7s should go into that unit.
Thanks Eli, I put in an order with Jim this afternoon.
I took a quick look at the plate curves for the voltages and currents on V2 and it looks like it won't matter much which tube type, just pick a good brand.
*Freudian slumber, plenty of sound
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