Let's talk vacuum tubes...
 

yesterday I picked up a used Ampeg SVT6 Pro hybrid tube /SS bass amp to get me a little more juice.

Ampeg: Pro Series - SVT-6PRO

This is one of the last Ampeg amps made in the USA and they are getting rare. However due to it's age and who knows how it was used / abused, I think re-tubing it may be in the cards. It needs: 4)12AX7 tubes and 2)12AU7 tubes. I know nothing about tube pre-amps so what brand should I be looking for at the best price? Should they be matched? all that stuff. Educate me please.

my brother does amp repairs.

check him out on FB - https://www.facebook.com/resonantelectronicdesign

Hey David, check out this guy. I buy my radio tubes from him and his prices are the best I've found. He also will match tubes for you before he ships.

FindATube.com: Vacuum tubes by mail order, vintage radio parts, antique electronics

*Disclaimer* I am into old radios, so the audio stage is the end of my signal chain. I am NOT an amp guy!

I think that having a good quality American or European brand is best. I don't use Russian tubes (except nixies) and would stick to Sylvania, RCA, National Union, and so on. I also buy from a couple of other websites if Bob @ findatube doesn't have what I need.

Antique Electronic Supply

Vacuum Tubes Radio Tubes - 5,000 different tubes in stock - Over 10 million tubes!

thanks Micah, I'll check him out. Cocker, this beast weighs 41 pounds, so shipping is not in the cards. Besides that tube don't travel well from what I understand. Thanks for input though and I will check your brothers page out.

Micah what other brands are US made?

I have a Univalve (single-ended Class A tube amp) and played with lots of 12AX7s (ECC83s). My cousin rebuilds old vacuum tube stuff and even makes low-wattage tube amps (he gave me a whole bunch of vintage 12AX7s to play with). To be brutally honest, I didn't see a huge difference between brand new Chinese 12AX7s and vintage Mullard ECC83s.

I've bought some stuff from these guys:

https://www.tubedepot.com/t/tubes/preamp-tubes/12ax7-ecc83-7025-cv4004

Fun to look around...

Power tubes are a different story. Lot's a good power tubes that make a huge sonic difference...

There were only a handful of actual tube manufacturers back in the day. Many brand names on antique tubes are just a stamp on the tubes mentioned above. If you find any tube with a major US manufacturer stamped on it (Like Zenith Philco GE RCA) it's probably made by Sylvania RCA Tung-Sol [New production made in Russia IIRC] National Union, Cunningham, or maybe a couple of others. I primarily deal with radio tubes (as mentioned) but have found the new-production foreign tubes to have a substantially shorter life over old 'Merican made tubes.

As a preamble, I am charged with maintaining my bands amps and tube preamps (currently six amps and three preamps) doing everything from tube replacements, re-biasing and tweaking circuits.
In the case of your amp, you have two tubes driving the tone circuit and two are preamp tube that drive the MOSFET transistors. The 12AX7 and 12AU7 differ only in their output gain with the 12AX7 being defined as 100%. The 12AU7 has a lower output of a relative 20%. In this scenario I'll hazard a guess the AX7 is the preamp tube and the AU7 is in the tone circuit.
There will be no need to have matching pairs in the tone circuit but matching preamp tubes will ensure balanced driving of your MOSFET's so I would pay that added expense.
As far as what to buy, well everyone is going to have opinions and right-wrong really comes down to what you want and what sonic differences do you hear. I personally like TungSol, Mullards, TAD valves. I have used JJ, Electroharmonix and even pilfered our campus legacy stock of original RCA and GE tubes. I would recommend going to the previously mentioned www.thetubestore.com - Your online source for audio vacuum tubes. or www.tubedepot.com, reading their evaluations and comparisons and match that with what you think is important sonically and what fits in your budget.

The power section of this amp is solid state MOSFET, so all the listed tubes are for the pre=amp only. There is a voltage adjustment knob on the amp so I can dial up more or less tube sound. I looked at an Ampeg SVT-CL all tube amp but it weighs in at a whopping 85 pounds and to re-tube it with say Winged C tubes is around $700.00. Too much weight, to expensive to maintain in my book. Besides that I play in a variety of bands with different styles of music. I don't always want a tube sound. It will be nice to dial up the tone I want.

here are the specs on this amp:


RMS Power Output (8 Ohms): 750-Watts
RMS Power Output (4 Ohms): 1100-Watts
Preamp: Tubes (2 x 12AX7)
Driver Tubes: 2 x 12AU7, 2 x 12AX7
Power Amp: Solid State
Tone Controls: Bass and Treble
Mid Tone Control: 4-Band Semi-Parametric
Ultra Low/High Boosts: Yes
Balanced Line Out: Yes, Pre/Post with Level and Ground Lift
Effects Loop: Yes
Preamp Out/Power Amp In: Yes
Dimensions (W x H x D inches): 19 x 3.5 x 15
Weight: 41 Pounds

JJ tubes are a goto for me as a regular working musician. They are of consistent quality and reasonably priced, made in Slovakia. I toasted my "golden ears" playing rock music for 40 years so fancy high $$ NOS tubes have no place in my kit.

https://www.tubedepot.com/t/brands/jj-electronic-tesla?gclid=CN3TkKK_jb0CFRFbfgodsiQALw

Amazon also has em.

so going to the tube depot I see the two tungSol tubes I would need.: 2) TS-12A7 and 4) TS-AX7 matched. Is this good?

what about these? http://www.ebay.com/itm/Ampeg-SVT-6-Pro-Preamp-Driver-Primo-Tube-Set-12AX7-/190649937589?_trksid=p2054897.l4275

I've used Doug's Tubes before. Tube-rolling is huge amongst a minority of guys. Your preamp tubes don't need to be matched, though you might want to find some that have low microphonics.

I am sure I am missing something but according to the block diagram in this manual, (www.ampeg.com/pdf/SVT6PRO.pdf) you have tubes that feed the MOSFETs. To me they would still be considered a pre-amp stage. The voltage adjustment you are speaking off directly adjusts the voltage across those tubes.

And I apologize, again according to that manual, you have 4x 12AX7's and 2x 12AU7's.

my buddy who is an FCC testing guy
and has fixed a lot of older amps
said tube preamps doNOT give the tube sound
that only comes from tube amps not the preamp part

correct:
Preamp: Tubes (2 x 12AX7)
Driver Tubes: 2 x 12AU7, 2 x 12AX7

So then what is the point of a tube pre amp section?

I strongly disagree with this. Power tubes are definitely important but I find preamp/reverb tubes to have a discernible difference in sound quality.

Putin's invasion may cut off our supply of special magik Russkie tubes