Lo-Gain still too much gain!

I have been running the "stock" Hi-Gain VTA board for years with 12AU7 Cleartops and have been happy. I rolled in several 12AT7's, and 12BH7's but settled on the Cleartops. Likewise with rectifier tubes. I would have never dreamt that rectifier rolling would affect the sound, but the NOS Mullard blows the JJ out of the water! I also settled on the JJ KT77 for the extended dynamics in the high end and slightly better bass response over the EL34.

Since my speakers are 102db, I have a Caddock Axiom Passive between preamp and ST-70. My friend and I did some listening tests and found the passive to be taking away details, so I broke down and purchased the Lo-Gain board. I also upgraded to the Triode PA-060-S Power Transformer as my stocker has recently started humming.

My first impression is the gain is not much lower than with the Hi-Gain running 12AU7's! I also noticed that the high-end is now a bit more harsh with my horns and midrange has gotten lost. This seems odd since I am still using the same tubes with the only difference being the driver board. My guess is the Orange drops were attenuating the higher frequencies? I still have hours to go to burn in the Auricaps, but first impression not so good.

Today I will start to roll some 12BH7's and "Old" Mullard EL34's to see if that helps. Any other ideas from fellow ST-70 aficionados?

System specs:
Rega P2 - Denon DL-103R
DIY Cinemag SUT - Bottlehead Seduction (Auricaps) - Amperex 7308's
Bottlehead Foreplay III (Auricaps) - RCA Cleartop's
ST-70 - VTA Lo-Gain (Auricaps, 2x100uF & 2x47uF Nichicon KX PS caps) - RCA Cleartops, Mullard GZ34, JJ KT77's
Klipsch RF-7's - Dean Wescott Crossovers

Those cleartops are something else, aren't they? Of all the tubes I've tried in the phase splitter position,s they are my favorite. Try a 6189 tube in the center position. It's military build (3 mica's with extra supports) with slightly less gain. Military build usually means sturdier with less microphonics, which I think makes a big difference in sound. Mullard GZ34 is classic and great (especially the Fat Base) but I'm using a HH Scott USA (probably RCA or GE), which seems to have a little more control for my amp. For the output tubes, I'm using NOS Mullard EL34's. I think they have a little more depth and detail than the current production.
Nice looking amp. You can get some hole plugs for those front sockets at Lowe's/Homedepot. They're chrome plated and usually in the parts drawers of the hardware aisle. Otherwise, look in plumbing supplies, where they're used to plug unused sprayer holes in sinks or in electrical to plug panel boxes. I prefer the plumbing version, which is "domed", the electrical version is kind of sunken in. Just personal taste.

I found that the feedback in the VTA low gain board is quite low. I couldn't get the full rated ST-70 35 watts at 1% distortion with the suggested feedback. If you lower the 7.5 K feedback resistor to around 3K, the gain will drop and the noise and distortion will too. I increased the feedback cap to 680 pf to get a clean 10kHz square wave without ringing.

However, at low levels with efficient speakers you shouldn't be driving the amp very hard and the feedback may not be the issue in what you hear.

Another thing to try is to slightly increase the output tube current by adjusting the bias. I found that the distortion increased rapidly for EH EL34s when the current per tube fell below 42.5 mA.

Finally, I found that the distortion dropped noticeably if the AC balance was set correctly. I measure the distortion directly, but Roy suggests another simpler approach. I wonder if some of the tube rolling differences that people hear result from never getting the amp optimally set-up with many of the tubes.

Best,
Dan

I seemed to have "tamed" the high end by tube rolling. The culprit was the RCA Cleartop in the driver stage! I rolled in about 6 variations including a 12BH7 and ended up with of all things an RCA black plate 12AU7. I did leave Cleartops in the phase splitter positions though ...

I will try modifying the feedback circuit to see if that cuts my gain down, but I think I am forever stuck with a passive between my active preamp as I have the preamp set to 50% and the passive rarely goes beyond 30%. Either that or I need to build a high quality (Caddock?) voltage divider between the RCA jacks and the driver board on the ST-70. If I do go this route, What input resistance does the VTA driver board prefer?

I am running the hi gain board with 12at7's, a Dared tube preamp that has a very hot output, and some Heresys specced for 99db 1W/1M. I dont think it has too much gain at all. Its perfect. But I also always listen loud.

CraigTone,
from your description, I would address first the likely problem of too much gain with the combination of Seduction and Foreplay, you are already cutting the gain down to 1/6th (50% & 30%).
Somewhere a few years ago I remember seeing a "mod" for the Bottleheads to reduce the gain, which essentially was just a resistor going into the volume control on the Foreplay. I don't know what your gain combo is with the Seduction and CineMag SUT but that could be high also.

Bob Latino wrote:Craigtone,
I don't think it is fair of you to complain that even the LOWER gain VTA board "has to much gain" when my personal feeling is that your complaint of excessive gain should be more focussed on your Foreplay preamp.

Bob

That is funny ... Bottlehead says the opposite. They say their preamp is a standard 10db gain and that my amp and speakers are too sensitive. I suggest you read Paul's whitepaper here: http://www.bottlehead.com/loosep/signals.htm Did you miss the fact that my speakers are 102db? I need probably 3W at most to make my ears bleed!

My point was that there was really no difference between the Hi gain running 12AU7's and the Lo Gain at all. Hoping for others that have been in my shoes to see what worked best for them.

To respond to Roy's comment - Yes, I am currently planning to replace the Sweetest Whispers with 10K 24-step mono Mini-V's to reduce the gain a bit more. I do see the problem on the CD as well as phono, so not isolated to the phono gain stages.

Right now it seems my best option is to order the 10K goldpoints coupled with upping the inline series resistors on the Foreplay and hoping I can get at least 30% of a volume control! Right now the first step of the Foreplay is too loud but the steps are non-linear on purpose until the 3rd step. I really want to keep the Foreplay as I noticed a huge increase in dynamics and soundstage switching from a passive to the active Foreplay as my preamp.

Found this:

www.diyaudio.com/forums/tubes-valves/96563-foreplay-iii-pre-amp.html

The real question in my peabrain is, why soneone would mate a 35 WPC amp to speakers of 103db sensitivity?

WntrMute2 wrote:The real question in my peabrain is, why soneone would mate a 35 WPC amp to speakers of 103db sensitivity?

35WPC is really not that much first of all. Most amps are 50-150WPC. Good luck trying to find an amp of lower wattage that has decent bass and/or doesn't cost an arm and a leg! I've tried ST30's and while I LOVE the sound of the EL84, the bass is non existent. If I wasn't a die-hard tube guy, maybe a First Watt.

It is all about working with what I have and building a system a piece at a time. When I started with the ST70, I had speakers with 89db sensitivity. I've upgraded just about everything since then. The amp isn't a big deal ... it is simply a matter of adding the proper attenuation at the right location.

Woo-Hoo!!! Problem solved! I replaced the 15K Sweetest Whispers with a pair of 10K Goldpoint Mini-V's and swapped out the series 33K input resistors with 100K on the Foreplay III. I can now put the attenuators at 50% (without the passive between the FP3 and ST-70) and I am just peaking 92dB at my listening position.

I am one happy camper tonight!

... Now on to some tube rolling ...