Trace Elliot VA400, VR350, & Hexa Valve

[tombowlus]

Okay, being the gear freak that I am, and having quite the Trace Elliot tube gear fetish, I have recently picked up a VA400 and VR350, and I have a Hexa on the way (already have a Quatra Valve, Twin Valve Combo, and a V-8). Yes, I know, out of control. But, I need some help from you guys/gals in the know.

The VA400, VR350 and Hexa Valve all employ what appears to be a similar tube output section. They all have six 6550/KT-88 tubes, and visually, there are a lot of similarities. But, not being the tube amp guru (and not being able to find a lot of information on-line), I was wondering if anyone had any information to share regarding the differences between these three. I do have the schematic for the VA400 (which I found on British Audio Service's website), but can't find one for the other two (and in truth, while I can somewhat follow the schematic, being a "layman" with regard to tube amps, it does not answer all of my questions). Any insight? Also, the VA400 and VR350 have a low, mid, and high setting for output power. Any ideas as to what exactly these controls do, and what the relative output might be at the various settings? I know that the V-8 allows you to run one or both banks of four output tubes (for 4 or 8 tubes), and I am wondering if this sets it for 2, 4, or 6 tubes?

Any tips or references on biasing? The VR350 and VA400 have bias adjustments (I don't believe the Hexa will, as my Quatra does not).

Finally, I have a question about the inputs on the VA400. It has a "valve preamp" input as well as active and passive inputs on the GP12 preamp section. My suspicion is that the valve preamp sends the signal to the driver tubes (like using the "instrument input" on the VR350), but yet the Gain and Master controls still have an impact on the overall volume (though the EQ doesn't seem to be in the signal chain at this point). So, my first question is what exactly is in the signal path when I plug into the "valve preamp" input. My second question is what exactly is in the signal chain when I plug into the "green" inputs on the GP12 portion of the VA400? From the schematic, it looks like I go through one half of the V1 preamp tube when I plug into the GP12 inputs, but I go through both sides of V1 when I use the valve input.

And of course, any general comments/observations/comparisons about any or all of these amps would be appreciated.

Thanks!

Tom.

 

[oddio]

Most of the useful information that I have on the Trace amps of that era came from Psycho Bass Guy. While we're waiting for him to make an appearance you may want to do an advanced search using his handle and the the amp in question. Plenty of good stuff in there.

The only other useful tidbit that I can offer comes from one of my phone conversations with Trace's UK service manager. He told me that the service technicians hated working on the Valve Series amps (Twin, Quatra, Hexa). The subsequent models were much easier to service.

 

[DEVILMAN]

IME, I can see why working on the Hexa style amps is a pain. Very tightly fitted internals. You have to take EVERYTHING apart to get access to the pwr sect & they have NO bias adjustability to see what's going on or to make changes...

my $0.02
~S~

PS: Tom B RULES!

 

[tombowlus]

While we're waiting for him to make an appearance you may want to do an advanced search using his handle and the the amp in question. Plenty of good stuff in there.

Lots of good stuff from Sir Psycho, for sure! He is an excellent resource.

Tom.

 

[John D Harris]

Lots of good stuff from Sir Psycho, for sure! He is an excellent resource.

Tom.

+1

We are very fortunate to have him around.

 

[oddio]

We are very fortunate to have him around.

Total agreement here. I had previously asked PBG's permission to quote some of his wisdom by proxy, so hopefully he won't mind that I do it again here. In a short period I was able to gather a little bit on the amps you asked about. Pulling this together in one place seems a worthwhile effort so here we go:

In reference to the VA400 (head) / VR400 (power amp):

The amp is ultralinear, which means it runs its tubes at VERY high voltages in a different way than most amps. You want to give it a 4 or 2 ohm load (there's switch on the back to select between them). At 8 ohms, you risk shorting your output tubes doe to inductive voltage spikes from the output transformer.

I also have a Trace VR400 and VA350. (...) and run VERY high voltages on their tubes and will eat crap power tubes in nothing flat. They're a total pain to tear down, but once you get them apart, they're the easiest amps to repair I've ever seen. I make it standard practice to swap out the plate loads and grid resistors in them for higher values, because they're always cooked (but totally functional) and it's a breeze.

Also, if you have time and know a good tech replace the grid resitors with higher wattage ones. The stock resistors are 1/2 watt and scorch pretty easily. I put two watters in and was done with it. They're all on the same board in the same place, so replacement is easy for anyone who knows what they're doing. If one should burn out, you'd lose the tube it feeds and possibly do some other damage, too.

I upped their wattage because some of them were scorched from using cruddy tubes. It actually makes it MORE dangerous if you have a problem, (lower wattage resistors will blow, acting like a fuse if there's a bad problem) but I keep a good eye on how my amps run. Also once scorched, resistors change, usually going higher in value.
I've never had to crack my V6 and I don't have schematic for it, but the VR400 and VA 350 both have protection relays on the main bus lines.

On tubes (related)...

...but no Tungsol 6550 could ever run at 730 volts plate and screen, like the set of Sovtek KT88's in my Trace Elliot VR400 has been happily doing for the past six years of road gigging.

Hexa...

The Hexa is pretty much the same amp as the VA350. (...) They're a biotch to get apart, but once you do, they're VERY easy to work on. You don't even need a skit for most repairs because the layout is so straightforward.

I'm not intending to cause death by quotation, but this is just a small example of some of the knowledge PBG shared in past threads...

 

[Psycho Bass Guy]

I do have the schematic for the VA400 (which I found on British Audio Service's website), but can't find one for the other two (and in truth, while I can somewhat follow the schematic, being a "layman" with regard to tube amps, it does not answer all of my questions).

Don't worry about it; it's wrong.

Also, the VA400 and VR350 have a low, mid, and high setting for output power. Any ideas as to what exactly these controls do, and what the relative output might be at the various settings?

It's a switch that lifts the cathodes of each respective pair of power tubes from ground, which leaves the tube in the circuit, but 'turns off' its emission. That's about the only simple way to reduce power level without getting into screwy impedance problems you would get if you just removed the tubes altogether. The cathode switch maintains the same primary impedance since all the tubes' plates and screen grids are still connected to the output transformer and your secondary (speaker) ratings don't change like they would have to if you just pulled pairs of output tubes: low power= one pair of output tubes putting out power; medium= two pair putting out power, and high= all six tubes putting out power.

I know that the V-8 allows you to run one or both banks of four output tubes (for 4 or 8 tubes), and I am wondering if this sets it for 2, 4, or 6 tubes?

Yep.

Any tips or references on biasing? The VR350 and VA400 have bias adjustments (I don't believe the Hexa will, as my Quatra does not).

Good tubes are MANDATORY for the VA400!

Finally, I have a question about the inputs on the VA400. It has a "valve preamp" input as well as active and passive inputs on the GP12 preamp section. My suspicion is that the valve preamp sends the signal to the driver tubes (like using the "instrument input" on the VR350), but yet the Gain and Master controls still have an impact on the overall volume (though the EQ doesn't seem to be in the signal chain at this point). So, my first question is what exactly is in the signal path when I plug into the "valve preamp" input.

You're bypassing the GP12 preamp, which is entirely s/s, and going directly to the preamp tubes.

My second question is what exactly is in the signal chain when I plug into the "green" inputs on the GP12 portion of the VA400? From the schematic, it looks like I go through one half of the V1 preamp tube when I plug into the GP12 inputs, but I go through both sides of V1 when I use the valve input.

The other side of the tube is for make-up gain for not using the GP12. I use my VA350 this way. The preamp doesn't sound as good as just going through the green input.

 

[DEVILMAN]

...hmmm, so the Hexa's pre tubes do exactly what if the pre is s/s? Does it act like an Eden Navigator & just the 2 pre tubes as a "tube front" to help color the input/sound to the s/s pre?

~S~

PS: Like Dice Clay once said: "Either ya are, or ya ain't!"...

 

[Psycho Bass Guy]

...hmmm, so the Hexa's pre tubes do exactly what if the pre is s/s? Does it act like an Eden Navigator & just the 2 pre tubes as a "tube front" to help color the input/sound to the s/s pre?

The GP12 can be completely removed and still leave a fully functioning tube power amp. The idea is that if you want the graphic EQ and pre-shapes, then you use the GP12 preamp, but if you want tube sound with no EQ, you use the other input. The tubes are AFTER the GP12 if you go through it.

 

[DEVILMAN]

Ok. Now, can I slave just the pwr sect of the Hexa from another pre source, totally bypassing all the Hexa's pre functions?


thanks
~S~

 

[Psycho Bass Guy]

yep

 

[popinfresh]

PBG - Are the plate voltages high on a V8 as well? Or not really.. If so, is there any better power tube for the job than others?

 

[Psycho Bass Guy]

I don't know about the V8. I've never even seen one in person. If anyone has a schematic, it should have voltages listed on it.

 

[tombowlus]

Good tubes are MANDATORY for the VA400!

Currently, it is loaded with JJ ECC83s's and (fairly new) Sovtek KT-88's, so I feel pretty good about that configuration.

Thank you for all of your input (past and present). Very helpful stuff!

Tom.

 

[Plain Old Me]

I don't know about the V8. I've never even seen one in person. If anyone has a schematic, it should have voltages listed on it.

NO WAY! Finally a tube head which PBG has never seen! It is very appropriate that the head is the V8 though...

 

[cfortner]

Very old thread, but I have a question.

I own a VA350, which worked fine, but one day he started to hum very loud.

Two techs tried to fix it, no way.

The problem seem to be correlated with the protection curcuit. I found a schematic of the VA350/400, but whithout the protection circuit.

Does anyone have a complete schematic with the protection circuit?

thanks a lot

Christian

 

[Wasnex]

Very old thread, but I have a question.

I own a VA350, which worked fine, but one day he started to hum very loud.

Two techs tried to fix it, no way.

The problem seem to be correlated with the protection curcuit. I found a schematic of the VA350/400, but whithout the protection circuit.

Does anyone have a complete schematic with the protection service?

thanks a lot

Christian

I'll send you what I have in a PM.

 

[Malletta]

Discussione molto vecchia, ma ho una domanda.

Possiedo un VA350, che funzionava bene, ma un giorno ha iniziato a ronzare molto forte.

Due tecnici hanno provato a ripararlo, ma senza successo.

Il problema sembra essere correlato al circuito di protezione. Ho trovato uno schema del VA350/400, ma senza il circuito di protezione.

Qualcuno ha uno schema completo del circuito di protezione?

molte grazie

cristiano

Ho acquistato un Trace Elliot VA400 rotto. Ho sostituito 3 resistori da 22 ohm sulla scheda che contiene il 12AX7, l'amplificatore rimane sempre in riscaldamento.
Ho lo schema elettrico ma manca il foglio 2 dello stadio finale, che contiene la descrizione delle protezioni ed attivazione dello standby.
Puoi aiutarmi?

 

[Wasnex]

FYI most of us do not understand Italian. Please write in English.

The VA400 service manual does not appear to include the protection circuit. The only reference I see is a couple of notes on the output section schematic that say the negative feedback also goes to a protection circuit. Also the Plate and Screen supply has relays that the protection circuit switches.

I have a VA350 protection circuit diagram. AFAIK the VA350 protection circuit schematic was reverse engineered. I don't know if it is 100% correct. I will PM both the VA400 service manual and the VA350 protection diagram.

 

[cfortner]

Would you pm me the protection circuit? I've problems with the VA350 in this area.