Dangers of working on a tube amp

Spyke

Getting my hands on a small tube preamp amp soon and seeing as it's used, I'm expecting a flaw or two and possible even a busted tube.

Even if it is in 100% working order I'll probably change the tube for something a little better.
My question is: how tricky is this to do? I know it's fitted with a 12AX7 preamp tube and the rest is solid state as far as I know.

What should I look out for? Would it need to be discharged and if so, how would one go about it?

cain

Its different but i used to own a Radial tonebone distortion pedal and it also had one 12ax7 tube. I just removed it with no problem or danger. You should be fine. Does it only have one tube?

Spyke

As far as I know yes, only one for the preamp.

AlanRatcliffe

No need to go to special lengths just to change a preamp valve - unplug it and pop the new one in. If you are going to be working on the circuit itself, you'll probably need to discharge the filter caps in the power supply.

cain

Yeah sounds right, tho it is sometimes an issue getting certain tubes. Last time i could only get sovtek tubes ... They all supposedly sound different.

Spyke

I won't go up leaps and bounds and splurge out on a NOS 12AX7 but a rudimentary upgrade to a Sovtek or something will help me thinks.

Thanks for the help guys!!

cain

Yeah, if you can get a ruby tube, try one of them, they supposedly lekker

Spyke

Francis Viviers wrote: Yeah, if you can get a ruby tube, try one of them, they supposedly lekker

Tell me more...
Where and what sort of pricing?

cain

As far as i know, their the same price as sovtek's. Im not sure where you can get one here, but ask around, else why not get some posted from overseas?

Spyke

I'm always a little apprehensive about shipping stuff from overseas - I don't mind but I've lost too much money due to out postal service. Lets not even start with customs and shipping costs.

I'll see what I can find. Thanks Francis

AlanRatcliffe

Spyke wrote: I won't go up leaps and bounds and splurge out on a NOS 12AX7 but a rudimentary upgrade to a Sovtek or something will help me thinks.

The quality of the valve is hugely important in a decent valve preamp, which are designs with lots of gain that will show up inferior valves fairly quickly. A good preamp can change character quite radically with different valves. Having said that, if it were a Behringer valve preamp, I wouldn't bother with getting too fancy - there is only so much that can be done due to the design.

If you want modern production valves for a preamp, look at the Sovtek 12AX7 LPS (R125 from Mr. Valve - cheaper than the JJs or other Sovteks!) and the Electro Harmonix 12AX7EH. I don't trust Ruby Tubes or Groove Tubes as they are usually just rebranding someone else's product.

Otherwise, good NOS don't have to cost a fortune - JAN Philips are plentiful, inexpensive, have a good neutral tone and are reliable. The standard by which all others are judged are the Telefunken, but they are expensive these days ($150 - 200 each) and fakes abound - so if you ever run across those at a decent price, grab them.

Spyke

Thanks for the info Alan.

Alan Ratcliffe wrote: <snip> Having said that, if it were a Behringer valve preamp, I wouldn't bother with getting too fancy - there is only so much that can be done due to the design.

How'd you guess? hehehehe!

I wasn't going to mention it because I know you and the Behringer brand! ?

Spyke

Ok, the amp in question is a Behringer Vintager AC108 which is a little 15w practice amp. I haven't played it yet but reading the reviews it seems to be decent value for money.

It runs a 12AX7 preamp and some suggest that this is a little too high gain and switching it for a milder 12AU7 or 12AY7 gives it more clean headroom.

I shall investigate!

AlanRatcliffe

Ah - when you said preamp, I presumed you were talking of a valve MIC preamp. Scratch the Telefunken valves then - a bit sterile for a guitar amp. The JAN Philips will still work well though, but I'd probably just go for a JJ or the Sovtek LPS.

Try a 5751 valve first to tame the drive, I've had great success with those. An AT7 brings the gain down (to 60 amplification as opposed to an AX7's 100), but I find it tends to neuter the tone and sparkle of the amp a bit in the process. The 5751s are closest to a 12AX7 in the circuit but lower gain (70). AY7 is a good match as far as the circuit goes, but the gain is way lower (17). Mr Valve has various NOS JAN 5751s for R170 ea.

Gearhead

I've little to add here, but if you are stuck between JJ's and EH the choice depends on what you want to play. If used for overdrive, the EH is audibly more agressive.

I also would like to add that putting in a valve with a lower amplification factor in a preamp is not automatically going to give you more clean headroom - it depends where the distortion comes from. Preamp stages are used for four different purposes:
- simply to work the voltage up to a level that the output valves need on their input;
- to drive a tone control circuit, so the voltage on the output of the gain+tone stage is almost equal to its input;
- to act as a phase splitter (which will not be the case with a standalone preamp);
- to act as a buffer (chances of that are zip with a one-tube-and-the-rest-solid-state amp).
For the following I am assuming that Behringer does not use the tube stages just for doing the tone circuit.

Let's now say that you want to use the preamp as your overdrive. The 12AX7 actually contains two triodes so it is mostly used as two separate gain stages. The first stage amplifies your guitar signal so much that it puts more signal on the second stage than it can amplify, causing it to clip (one can debate about the tonal qualities of triode clip, but that's a different story). The maximum voltage swing a triode can make is determined by the plate resistor, the plate voltage and the current through the tube.
Changing the tube to have a lower gain and not using the amp as overdrive will put less voltage on the input of the second stage, so it does not clip as much (all other things being equal). It does not change the maximum voltage swing of the second stage and therefore not the clean headroom.
What would increase the headroom, is increasing the plate resistors and changing the tubes to have a lower gain. A higher plate resistor gives more voltage swing for the same current change so it can make up for lower tube transconductance. It however needs a fair bit of calculating and testing to see if the rest of the circuit will appreciate this change.
Dumble for instance used higher-than-standard plate resistors with appreciable success.

Spyke

Thanks all!

@Alan - I'll look at the 5751s. Thanks for the info.

@Gearhead - I understand what you're saying (surprisingly enough!). I'm no electronic engineer but I understand the theory (and your great explanation thereof) I think I'll stick to a (slightly) lower gain tube first before getting ambitious. The rest of the board may not do the plate mod justice.

needleshy

i also have a tonebone and could clearly hear the difference between valve changes......

the ruby that was in there was ok but it seemed to lack cream so i changed it to a sovtek adn made changed the sound and made it a little tighter, i recently purchased a JJ and that's by far the warmest and smoothest of them all.
just a pity my true bypass switch found a fault and the pedal has been with radial for the past month...... but they gave me a loan pedal to use in the the meantime while we play shows..... haven't found service like that in a while here.