Recently upgraded ST-70. Where'd the bass go? (Heresy I

[therobbstory 5]

Long time reader, first time poster. 

 

So in the last month, I've acquired a pair of very nice Heresy I speakers dating back to 1984. I've also picked up a Dynaco (kit) ST-70 which was in pretty rough shape. I immediately rebuilt the crossovers with a Crites kit, and rebuilt the entire ST-70 with a Bob Latino VTA 6SN7 octal kit, retaining only the trafos, quadcap, choke and rectifier tube. 

 

Prior to the amp rebuild, sound was decent. Full-bodied, good sound stage, and listenable bass. Post rebuild, everything sounded better. Exception: the bass was gone. Like someone took the slider on the far left of the EQ and pushed it all the way down. I listen to a lot of jazz which is what informed the purchase of these components. 

 

Two suspected factors:

1. The 6SN7 is a power hungry tube. I may be starving it a little by keeping the stock transformer. The trafo gets hot to the touch, and there's some mechanical hum which can be heard through the speakers. I've ordered a new one from Triode.
2. Output tubes are cheap Chinese EL34s I got off ebay for $30. I just wanted to get something plugged into the sockets to make sounds come out of the speakers. I'll likely order some Gold Lion KT66 based on Bob Latino's glowing (heh) review.

Am I on the right track here? Speakers are on Crites risers about 6 feet apart, maybe a foot away from the wall, and about 5 feet from my listening position. These are the first horns I've owned, and I'm LOVING the mids/highs. Before I run out and buy a sub, I'd like to know what I can be fixing first.

Edited August 11, 2016 by therobbstory
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[tube fanatic 1186]

So, if I understand correctly, the speakers sounded ok after the xovers were rebuilt, but prior to the amp upgrade?  Did you change the EL34s prior to the upgrade, or after?  Was the amp biased for EL34s, or some other tube?  The 6SN7s are not all that power hungry as they don't draw much current in the context of things.  The factory power xfmr should be fine in spite of running on the hot side.  Have you verified correct operating voltages?  As far as the hum goes, I'd replace the electrolytic caps.  Mechanical hum from the power xfmr should not be audible in the speakers.  The simplicity of the ST-70 should make troubleshooting fairly straight forward.  One last point is that it is not a good idea to have the amp sitting on top of the speaker.  Even slight mechanical coupling can cause microphonic tubes to do some bad things.  I'd move the amp.

 

Maynard 

[erik2A3 139]

The ST 70 is an outstanding match for Heresies.  In addition to the above suggestions, I would retrace every single connection you made in the modification of what was previously a very decent amplifier.  I am not clear on whether you used the modified amp with the original crossovers -- meaning that it would be good to recheck your crossover connections, particulalrly in the low-pass section of the network.  The 6SN7 really is not a "power hungry tube;" I have built many an amp and preamplifier with it without any such problem.  Nor should the brand or country of origin necessarily be suspect in the anemic bass response you describe. 

 

Also check your speaker connections for correct phase, both on the input terminals on the cabinets and (more specifically) on the crossovers.  A sub can always be a nice addition, but if you are not hearing the low end response you heard prior to the modifications you made, something else is amiss that needs to be addressed first.

[mike stehr 975]

Just a thought...but maybe those Sonicaps accentuated the midrange and highs, tipping it up over the bass a bit.

[erik2A3 139]

If there was an assembly manual that came with the modifications you did on the amp, have a look at it to see if it (just possibly) reverses absolute phase (aka polarity).  If it does, you will need to reverse the polarity of the speaker connections at either the amplifier end or speaker terminals.  

[therobbstory 5]

So perhaps my post was made a bit in haste. I cracked open my preamp today, an AMB Alpha10 which I had to finish wiring the I/O on. Since I had it open, I took the buffers through the initial setup. I was surprised to find the bias WAY off on both channels. I must've been a few glasses of wine in when I built it and not gone through setup. So that, coupled with about 10-hours of burn in on the EL34s changed the sound dramatically. MUCH more balanced through the frequency range, and a real joy to listen to now. The girlfriend was so excited, she blew $150 on vinyl today!

 

Thanks for all of your thorough replies. Speakers are correctly wired in phase, and I've moved the amp from the speaker onto some rubber isolating feet.

 

Man, I'm pleased as hell with this setup. 

[erik2A3 139]

Great!  Heresies and a Dyna 70 are really nice together!

[therobbstory 5]

Thought I'd pop in with a quick update. I've installed a matched quad of Gold Lion KT66 and WOW. It's a whole new beast. Incredibly dynamic, tight and controlled bass, forward mids, and crisp highs right out of the box. I think I'm leaving this one alone for now.

 

Will post some photos once I get my living room rearranged.

[JMON 428]

The Heresy does not go very low. One thing that will really help the bass is to move the speakers into the corners.  On the floor or at the ceiling would be best. Doe some experimentation and see what sounds best to you.

[therobbstory 5]

8 hours ago, JMON said:

The Heresy does not go very low. One thing that will really help the bass is to move the speakers into the corners.  On the floor or at the ceiling would be best. Doe some experimentation and see what sounds best to you.

So this is more or less where they're going to live. We've got some better furniture on the way (Mapleshade LP shelving should be here today) and the whole system will live here. Not the most ideal, I know, but I'm not the only one making decisions in this house. Getting really nice imaging, soundstage, and no longer having the anemic bass problems.

 

[NOSValves 542]

I'd spread those speakers out and toe them in a bit. Get them closer to the corner on the right and closer to the LP rack on the left.  

[JMON 428]

10 hours ago, NOSValves said:

I'd spread those speakers out and toe them in a bit. Get them closer to the corner on the right and closer to the LP rack on the left.  

That's what I was going to suggest, then I saw his post...

[karlson3 74]

have you tried them on the 16 ohm taps? - that's where I run my Heresy Is with a Chinese SE EL34UL amp

[Stump 63]

Just a thought regarding the power transformer getting hot. These amps were designed for 117 VAC, our power today is 120-127 VAC in most areas. Setting power tube bias around 1.2 volts or lower, instead of 1.56, seems to help and I've not noticed any negatives in the sound, I have an all stock ST-70 and Heresy 1's. 

[NOSValves 542]

lowering the bias is a good idea for todays wall voltage! In fact its not a bad idea even if you have 117VAC...Modern production tubes even at the specified voltages are not to happy at the original bias point.