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Vintage SVT (MTI) biasing procedure?

boroman

boroman

Apr 27, 2009
374
149
boro
www.dustblow.com
#1
Guys, can I use the test points without the scope?

I mean I biased a dozen of amps using kind of Weber bias meter (attached between tube and a socket) + voltage meter and calculated bias from known formulas. And wanted to know if the SVT's test points should show the same readings connected to voltage meter (as the weber "between-tube" thing)? I know that better give that to the tech, but sometimes on the road you must do it by yourself.

I've read somewhere that the sweet spot it's like 24ma per tube (73ma per side?), so does my meter will show "that" values or that test points are calibrated differently for volts or i don't know? Also, been told that MTI SVTs due to other transformers was 350watts instead of 300 (magnavox era), so this has something to do with plate voltage or different bias than 70's SVT's?
 
#2
Hi.

boroman said:
Guys, can I use the test points without the scope?
Click to expand...

Yes You can, that's the reason behind 'em being there in the first place.

I've never biased an older SVT, but according to the instructions, the reading will be the usual mV not mA.
The reading will show (roughly, but accurately enough) the same figure though, depending on the inaccuracy of the indirect measurement circuit.

Regards
Sam
 
#3
The test points use the same circuit as a "Bias King" uses. A 1 ohm resistor between cathode and ground. mV=mA on the cathodes. Having this resistor as part of the build also allow one tube tube to drop out of circuit if a major failure happens (resistor will burn out). I installed them in my V4B and added test points as well as a bias pot. At that time they were not as prized as today un-modded?
 
#5
Which color touching stick on the meter goes where?

:D

I feel like a big dope asking this so I worded it for dramatic effect, but I'm smart in other ways that don't pay as well. Got A's in English.
 
#6
Thanks

So 72mV is for the each trio (V1,V2,V3 and V4,V5,V6?) and... is it considered as "cold"? I want SVT to scream a little more but not that way I'll need to replaced whole set in few months ;)

Using the standard bias calculations, 700V pv and 42 watts max plate dissipation for 6550 - the "optimal" (70% dp) point is reaching 42mA/tube... so a LOT more. I remember having the same issue on the Orange AD200 amp. Can anyone explain? Are they running so cold there?

I wanted to buy Sovtek 6550WE or JJKT88s (but I think JJ will give me more clean power which i don't want...). Is this a good idea?
PBG did not recommend Winged C's - they're also pretty pricey these days. He recommend tung sol 6550s but I bought 2 sets in my life for other amps (svt-cl and orange) - both sets was bought from other companies and in different time - on time was bad tube in the set (weird parameters), next time bought a sexted and one failed me after two hours of playing. So i'm not buying this crap again.
 
#7
The bias suggested is safe range and provides long tube life. "Cold bias" will make the amp sound a little thin and late to breakup up. "Hot" bias runs the tubes hotter and shortens life but it breaks up sooner (excessively hot can cause red plating, avoid this at all costs). Between those two extremes is the "playground", I personally use the distortion notch method to find the "perfect" balance point. From that point I can shift from a little colder to a little hotter depending on what the owner wants the amp to do (but I always recommend the first time they play the amp at that balance point as IMHO they sound best there).
PBG and I disagreed on Winged "C"s, but you sound like you want early breakup so IMO go for the Sovtek.
 
#9
boroman said:
Are they safe with SVT's 700 PV?
And what setting should be considered as "warm" when biasing SVT?
Click to expand...

You (or actually I) need to know the starting point for the tube set. If that point is 0.75ma (meter reads 0.75mv) then the more the current draw the "hotter" the bias I would start listening tests at 80ma (always keep an eye on the tube plates when going hotter). If the point is reached that you like but you see some red starting on the plates BACK down.

700Vdc sounds a little high to me? Is this the measured plate voltage with tubes loaded? I have not personally seen Winged "C"s flash over, can say about the Sovteks as production runs had varied so much that I personally have not tried them in about 3 years. Controls have gotten tighter.
Measure your actual plate voltage loaded with valves, no signal (cab or dummy load attached), use locking clips of some kind so connection can be done with power amp unplugged. Then power up the unit with your hands free of the unit and test probes (make sure your meter is rated to 1KV input max).
 
#10
boroman said:
Thanks

So 72mV is for the each trio (V1,V2,V3 and V4,V5,V6?) and... is it considered as "cold"? I want SVT to scream a little more but not that way I'll need to replaced whole set in few months ;)

Using the standard bias calculations, 700V pv and 42 watts max plate dissipation for 6550 - the "optimal" (70% dp) point is reaching 42mA/tube... so a LOT more. I remember having the same issue on the Orange AD200 amp. Can anyone explain? Are they running so cold there?
Click to expand...

it tells you the spec to set it at right on the front panel (the instructions are silkscreened on the amp's chassis). it's 72mv and it isn't running them cold due to the circuit, and you don't need to calculate anything.


boroman said:
I wanted to buy Sovtek 6550WE or JJKT88s (but I think JJ will give me more clean power which i don't want...). Is this a good idea?
Click to expand...

sovteks are fine, but the JJ's don't sound any cleaner to me in them.

boroman said:
PBG did not recommend Winged C's - they're also pretty pricey these days. He recommend tung sol 6550s but I bought 2 sets in my life for other amps (svt-cl and orange) - both sets was bought from other companies and in different time - on time was bad tube in the set (weird parameters), next time bought a sexted and one failed me after two hours of playing. So i'm not buying this crap again.
Click to expand...

i'm not a fan of the overpriced winged C's either. my favorites are valve art KT100's in them. IMO, the tung sol's are fine, as are the JJ's or the Sovteks.
 
#11
Thanks. I have no way to measure PV in mine (meter goes to 500V), but I;ve read that typical 70s SVT PV is 670-695V. So if the MTI is putting 350watts clean so I thought it probably has a little more PV due to different transformers at that time?

John_k: There's one other option. The unpopular JJ6550's - have you tried these? I need more midrange than "extended" range like JJKt88 has (from my tests). So maybe their 6550s will be fine? I find JJ's a little sterile but great thick glass and can easily handle "troubley" vintage amps.

By the way, as we are here speaking on the tubes, what do you recommend for the preamp/driver? I heard that driver tube is probably the most important in that amps and it really can change the overdrive point/headroom thing...
 
#14
The tubes that you mentioned have all been put into service in vintage SVT amps. The JJ is a good tube.

You have be careful when interpreting the numbers on data sheets. The maximum ratings are dependent on how the tube is used in the circuit. If you click twice on the tube images here you can view the construction in detail for several makes and compare them to some extent. I would contact this company, tell them what type of amp you have and how you want it to sound, express your concerns about the plate voltage and the ratings of the different tubes and see what they have to say. They know what type of returns they get for a particular make and model of amp. At the very least listen to what they have to say and then decide if it makes sense.

When you decide on a particular make, keep in mind that all matched sets are not to the same spec. Since you want early distortion, you can specify that. They will select a tube set that matches at a lower plate voltage and transconductance. A set that is matched at a higher plate voltage and transconductance will provide less distortion and more headroom. Again, ask the tube vendor these questions and see what they say.
 
#16
I like NOS Philips, Bugle Boy, or RCA, but that is my personal preference. There are lots of great sounding tubes and I tend to buy what's available when I am stocking up. They are expensive. The original SVT's used Mullard. Of course Telefunken sounds amazing but can cost more than any other tube.

There are great long plate and great short plate tubes. In general, there are a lot of variables that affect the characteristics of a tube. Different makes will be more musical than others, break up differently, and have different physical characteristics that affect microphonics. Sometimes you simply need to try a bag full of tubes to find one that gives you what you are looking for because the supply isn't consistent. If you can find a hamfest (a ham or amateur radio fleamarket, see arrl.org in the US but every country has organizations like this) in your area, you can find great deals on tubes. I like to buy new and used tubes at one or two dollars each and try them. It cost next to nothing to buy a bag full.

The early SVT's had a 12DW7 before the 12BH7's. You can use a 12AX7 instead of the 12DW7. Other SVT's use a 12AX7 as standard in this position. Of the 12AX7's, some will distort sooner than others. Some have higher gain than others. You can search for high gain 12AX7 to find what is currently available. I don't know about Invalid Link Removed vendor but they specialize in 12AX7 tubes and their different characteristics for musical instrument amp applications.

A more affordable option might be to try different pedals at the input. Some like the VT Bass but there are a lot available to experiment with.
 
#17
Well from what I've heard, long plate ax7 on inverter (i mean that tube which steers 12bh7's) will give me more clean power than short plates - in general.

Speaking of the input, if the amp stay too much clean I use Lehle Sunday Driver all the way up. between bass and the amp.
 
#18
I have a matched set of TAD 6550s in mine, and am extremely happy with their tone and longevity so far. They might be an option for you, IIRC you live in Europe?

They select, match and burn in Chinese tubes, nd have a good reputation. I originally had NOS GE tubes from Watford valves in the UK, but they drifted like crazy and I have them sitting in the boxes as spares.
 
#19
One Drop said:
I have a matched set of TAD 6550s in mine, and am extremely happy with their tone and longevity so far. They might be an option for you, IIRC you live in Europe?

They select, match and burn in Chinese tubes, nd have a good reputation. I originally had NOS GE tubes from Watford valves in the UK, but they drifted like crazy and I have them sitting in the boxes as spares.
Click to expand...

Are you referring to that cheap 6550A-STR they have? I thought about them... but bought Sovteks.

I bought the best EVER 6L6GC-STR from TAD for my V4B they were super clean loud, punchy, and *that* sound. JJ7027A was like tiny crap in comparison. I've dealt with them few times and quality of their tubes is outstanding (as the matching!).

About Watford Valves, last year I ordered sextet of TungSol 6550s and one tube failed after playing few hours. That was crazy..
 
#20
boroman said:
Are you referring to that cheap 6550A-STR they have? I thought about them... but bought Sovteks.

I bought the best EVER 6L6GC-STR from TAD for my V4B they were super clean loud, punchy, and *that* sound. JJ7027A was like tiny crap in comparison. I've dealt with them few times and quality of their tubes is outstanding (as the matching!).

About Watford Valves, last year I ordered sextet of TungSol 6550s and one tube failed after playing few hours. That was crazy..
Click to expand...

Yes, that's the one. I was also very happy with the cap job and service they did on my SVT.
 

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