max plate voltage question for 12ax7,...please help.

[tonequester]

Greetings. tonequester here.

I need to find out approximately how much over spec any of today's major amp manufacturers are pushing 12ax7/7025 pre amp tubes. This concerns all tube designs only, and in any price category. I would be especially interested if any manufacturers exceeds 450 volts to the plate. I knew that such tubes have been "pushed" over spec, but did not realize until today that vintage Fenders have used up to at least 440 Volts(that by what schematics I have). Tubes truly ARE forgiving ! Any help with this would be appreciated greatly, as modern schematics are so hard to come by these days. I couldn't even BUY a schematic from Blackstar, for the HT-1R practice amp I paid $250.00 for, and have had no end of problems with. I've HAD IT ! I'm researching designing my own. If I'm going to have to pay $800.00 plus for a low power, hand wired and soldered amp, I will build it myself. I might as well try to design it as well.

Thanks in advance, to anyone willing to spend the time to reply to my post. I welcome all comers, and greetings/best wishes to all(this IS my first post here, I'm a newbie to forums, but I understand that THIS is where it's at ! tonequester.

[Jazz P Bass]

Well that certainly is an open ended question.

You need to decide which amplifier function you want the tube in.

There are reasons for raising or lowering the plate voltage on a preamp tube.

At a higher plate voltage the circuit will have more 'headroom' & less distortion.

A good place for the first tube in the signal chain.

I would not go over 330Vdc on a 12AX7.

Plus, it is not as simple as the plate voltage only. The cathode voltage also plays a part in the circuit.

[tonequester]

Greetings Jazz P Bass, and thanks.

I appreciate your input, and I do not plan to exceed 330 Volts. I was just rather shocked at the values for vintage amps that I was checking out, and as many go over 330V., as go under. In doing this project, I just want to understand as best I can the mentality of both vintage design and modern design, plus I'm just plain curious as to how far the venerable 12ax7 has been pushed over spec in this regard. I truly appreciate your opinion, especially when I believe that it is good advice, and it IS !

Glad to "meet" you here on this forum.....Best Regards, and good luck with any of YOUR projects ! tonequester.

[Enzo]

Yeah, but is that 440v you found on the TUBE or just the supply? A 400v supply is not stressing the tube if the plate voltage winds up at 250v. And as JP says, i also matters what is going on at the other end of the tube. Remember, if you are 10,000 feet in the air without a parachute, you are only in trouble if you are not also in an airplane. 300v on a tube plate and 100v on a cathode is no different to the tube from 200v on the plate and zerov on the cathode.

I don;t agree that schematics are hard to find. SOME are hard to get, but mostly they are available.

[tonequester]

Greetings Enzo.

I want to thank you for answering my post. Truthfully, I had hoped for your input, which is always welcome, even if the straight "dope" is not what I had hoped for. The truth is, that I am always getting in over my head, but I have a driving curiosity that's an inseparable part of who I am. The 440V. spec that I quoted was for sure the plate voltage, as I did run into numerous schematics that only gave the supply voltage. I pretty much just didn't count any that were obviously supply voltages only, and some schematics that I have, do not even give voltage readings. Point well taken about the relationship between plate and cathode voltages, and what a tube "sees". This is the thing that ticks me off about schematics, at least vintage schematics. They seemed to be made up to no particular standards. My Hi-Watt schematics are terrible scriblings, freehand drawings that look like a 7 year old drew them. I thank you VERY much for generously sharing the Blackstar schematic. A quick glance at it tells me that no matter what I think about my experience with them, they at least make a useable schematic. I will spend considerable time studying it. The third HT-1R, which I just fixed myself is still playing fine, and with the trouble I had getting a working amp, the whole experience taught me several good lessons concerning the way business is done in the amplifier trade these days. You taught me that entry level is an important term, even when sales pitch says studio/practice. At least they make a good schematic for the HT-1R. Just to let you know, I'm currently studying for that C.E.T.a exam, but I am studying the Navy(N.E.E.T.S.) course online first. I'm not looking to be a tech for anyone but myself. I've just begun A.C. circuits, so I have a long way to go, but I will always experiment. I have built a MOD 102 tube amp kit now, and learned considerably doing so. Now I'm trying to prepare myself to build my own design, which I will admit is pretty crazy for my level of non-expertise. In the coming weeks, you will probably run across quite a few of my strange posts. I welcome you to pipe in with no holding of punches, I've developed a thick hide in my forum experience so far, so I can take it. At first, I thought you were a pretty gruff guy, but now I think that you must have answered a thousand bizarre posts, many of which were repeats upon repeats. Now, I tip my hat to your patience, and wonder what keeps you coming back. The only answer that I can think of, is a great love of electronics. You are really a big asset to any forum you participate in, and that ain't no kissy butt B.S.

Thanks again, in all sincerity and best regards Enzo. Have a Happy Thanksgiving and a Merry Christmas. I know............HUMBUG ! (humor intended)

[Jazz P Bass]

And for the record, the Blackstar HT-1 is a hybrid amplifier.
Most of the signal conditioning is obtained by the use of opamps, not tubes.

So if you want to build a real tube amp, copy something else.

[tonequester]

Greetings Jazz P Bass.

Thanks for the reply. I realize that the HT-1R is a "hybrid" amp. What I didn't realize when I bought it, is the fact that the solid state components were actually used in the signal path. I had asked the salesman/dealer to show me what he had, in an affordable tube, amp just for practice, and was led straight to the Ht-1R. I should have done some research before purchase. I usually do, and will ALWAYS form now on. It took me three amps to get one that worked properly. # 1 blew up at 5 months, #2 came with a non-functional reverb, and #3 had a speaker that was only mounted to one of the three mounting studs, had a punctured speaker, and interior cabinet damage. So much for Namm's "Amp of the Year" title. I fixed #3 myself, and it is still o.k. after about 8 months of daily use. I am convinced to go all tube now, and this is the reason that I turn to forums such as this one. They do so, at times speed up the learning curve. I am not only a newbie here, but a neophyte where electronic theory is concerned, but I am too curious, and have too much fun to quite trying things that may be over my head. This also keeps up the "hands on" skills that I have honed.

Thank you sincerely, for your time in replying. That is most appreciated. tonequester.

[nashvillebill]

Here is one site with a ton of schematics, Fender as well as others. Schematic Heaven. Where All Good Amp & Effect Designs Await Resurrection...

I'm not a Fender expert, well actually hardly an expert on anything to be truthful, but I haven't seen any Fender stuff (ort anything else) that has 450 volts plate-to-cathode on a 12AX7. Their power supply (B+) might be 400 volts or more, but it's dropped considerably by the time it gets to the preamp tubes. Even the Ampeg stuff I've fiddled with has less than 300 volts on the 12AX7 plates.

A good site for beginner tube amp builders is AX84.com - The Cooperative Tube Guitar Amp Project They have a lot of projects that are well documented--schematics, chassis layouts, building instructions, etc.-- ranging from very simple to advanced.

[defaced]

I would be especially interested if any manufacturers exceeds 450 volts to the plate. I knew that such tubes have been "pushed" over spec, but did not realize until today that vintage Fenders have used up to at least 440 Volts(that by what schematics I have).

Can you post a copy of this schematic?

[tonequester]

Greetings nashvillebill, and thanks for your reply.

The vintage schematics that I have looked at (89 in all), are in no way standardized. However, there did seem to be a difference in the way that power supply voltages and plate voltages were represented. I may be wrong, I am so often, but what I took to be plate voltages were printed,or drawn right next to the schematic symbol of the tubes plate, while the power supply voltages were shown much farther away from the schematic symbols for tube and plate. They were much closer to the power transformer. If I am not correct in my observations concerning the vintage Fender,Gibson,Marshall,Ampeg,Hi-Watt,Magnatone,Vox, and Sears Silvertone schematics that I looked at, I wonder just how anybody ever figures them out. Just to throw it out there, Hi-Watt has the worst schematics that I can conceive of. They are hand drawn with the talent of a 5 year old, with no values on anything but resistors and caps. I don't see how their own techs ever fixed any of them, unless they had "special" info stashed.

My main reason for posting this question had to do with curiosity more than practicality. I wouldn't push design specs beyond the tube manual's suggested max value of 330 V., for a 12ax7. I WOULD be interested to know if any modern amp manufacturers push the 12ax7 beyond maximum spec, and even how much as an example. This concerns the old adage that "tubes are forgiving", that I've heard hundreds of times over the years. I'm interested to know just how "forgiving" they are, and to see if the trend that I noticed for the values to increase over the years from the 50's to the 70's(as noticed on my schematics), has continued to this day. Just trying to keep up with the evolution of the all tube amp.

That being said....Thanks again for your input, and a very special thanks for the thread to Schematic Heaven ! Best Regards, and Good Luck from tonequester.

P.S. Glad to "meet" you here on the forum !

[nashvillebill]

Well, I dug through some of the Fender schematics--and what do you know, I did find a couple with high plates. The Vibroverb AB763 for example does show 400V on the plate of a 12AX7, but I can't make out what the cathode voltage is.

[tonequester]

Greetings nashvillebill.

Thanks for the reply. I too had some trouble reading some of the values. I take it that I was correct in my believing the values, that were right next to the tube plate schematic symbols, are indeed the plate voltages, while those found considerably closer to the power supply to be power supply values ? Enzo graciously sent me a thread, so as to download the schematic for my Blackstar Ht-1R(He is an authorized Blackstar tech). I notice that on the Blackstar schematics, voltages are given at the closest capacitors to the plates, along with the cap value. I wonder if modern schematics are now at least more or less standardized ?

I am still curious as to just how much plate voltage has been pushed upward on the 12ax7, on any major brand of modern, all tube amp. LO and BEHOLD ! I just looked(with magnifying glass) at the Blackstar HT-1R schematic, AND IT READS 400V., at the cap closest to the ECC83 plate. The cap value is a puzzle. It looks like this: 22n. I don't know if this is nanofarad or not. I've always seen that value like this : 22nf. Perhaps Enzo could clear up this foggy picture, I could be wrong on everything for what I truly know and understand.

One thing is for sure, I need a class on how to read schematics in detail, although I have never run across, any such "animal" !

Thanks again for your ongoing interest, and input. it is greatly appreciated. Best regards, and luck in all of YOUR projects, etc. tonequester.

[Jazz P Bass]

That has gotta be on the reverb circuit.

The tubes are in parallel.

[nashvillebill]

... voltages are given at the closest capacitors to the plates, along with the cap value. I wonder if modern schematics are now at least more or less standardized ?... I just looked(with magnifying glass) at the Blackstar HT-1R schematic, AND IT READS 400V., at the cap closest to the ECC83 plate. The cap value is a puzzle. It looks like this: 22n. ....

No, in this case you are seeing the voltage rating of the cap itself, not the voltage that's actually on it. On your Blackstar schematic, C11 and C117 are rated 400 volt. C13, close by, is rated 1kV.

The schematic does not appear to show plate voltages or the "HT Pre" voltage, but it appears they tell you that B+ is 290 volts (look at the power supply section, right after the diode D4) Notice the caps C64 through c67 have a voltage rating of 500 volts, by the way.

Yes, 22nF would be 22 nanofarad.

[Enzo]

Yes. +290v is the highest B+ node, so the preamp supply is lower than that. Your tube doesn't see even remotely 400v.


To be honest, as a technician, I don't much worry about the voltage at each spot. COnsidering the B+ starts at 290v, I would expect something like 150v on the plates of your tube. If I found 170 or 125, I would say "in the ballpark" and move on. This is a little guitar amp, not a NASA space probe. Nothing is critical voltage.