Teach me about pre amp valves

musicegbdf

OK , having replaced my power valves (see which 6V6's) , I got to thinking about the pramp valves.

It has ECC 83S , which I think are also called 12AX7's., but I know there are other types. Are they swapable , or does it depend on the Amp. What are the different characteristics of them.

My Amp has 3 of them .. teach me oh masters of the valve...

Van_Hayden

In the Ashdown I'm guessing?

musicegbdf

Yep....

Catthan

there are many options with the nos ones or old ones being the most desirable if your ears can tell the difference.
I ve tried ecc803s irrc and 5751 in my Badger in replacement of the ecc83. kept the latter in v1 and put the 5751 in the pi. from the little digging I ve done, the influence of a tube may depend on the circuit topology of each amp, meaning that some could sound insensitive to different tubes. there many experts here who can shed a light on this and offer you more specific options.

Van_Hayden

V1 and V2 are doing both signal and reverb duties, most difference with V1. But using a lower gain valve will reduce your reverb. Phase inverter isn't the usual design, but you can mess with it.

Voltage isn't that high so it's kind on valves.

Van_Hayden

V 1 is closest to the input btw...

crunchman

5751 can normally be used in place of ECC83/12AX7 types and will reduce the gain.  In most cases you can experiment with 12AY7, 12AU7, 12AT7 and their ECC equivalent types as well but there may be exceptions to this for some circuits.

ecc83

ECC83/12AXY: High gain, high impedance double triode. The most commonly found valve in guitar amps. There ARE sonic differences between brands but you need a careful A/B test to prove it. Those that claim "night and day" changes have never, to my knowledge ever posted any valid data...Like not ONE second of a .wav A/B test. The TAD 7025 hg is a decent value valve I have found.

ECC82/12AU7: Low gain, low impedance double triode. Often used with both sides paralleled as a reverb driver. Used in the very best amps(! cough!) as a low impedance op valve driver. NOT ever to be replaced with an 83 or 81(they don't have the power rating).

ECC81/12AT7: Medium gain, medium impedance double triode. Actually an RF/TV valve and pretty ghastly in audio applications. The few I have tried have been miccy as hell. 

These 3 have the same pinouts and heater ratings and CAN therefore be swopped but a very few  circuits will object!

12BH7: Found in some big ass bass amps as power valve driver and of course MOST famously in the HT-5.

Dave.

ICBM

Be careful with the voltage ratings of some of the types in some circuit positions, especially cathode followers (eg V2 in classic Marshalls and modern Voxes). You can't safely use a 12AT7 or 12AY7 in these. A few other substitutions can cause trouble as well, either to the valve or more rarely (but not never) to the amp.

12AT7s are actually used quite commonly in guitar amps, mostly as reverb drivers and phase inverters - they *can* be used in gain stages as well, but often don't sound very good (somehow 'cold' or 'sterile') and can be quite microphonic as Dave said.

12AU7s are used only rarely but can be a very useful substitution for a 12AX7 if you have an amp with a *lot* too much gain - they make a very big difference, usually too much in most amps, but in some like the Peavey Classic 20 they can make an amp with stupid amounts of gain quite nice and bluesy.

12AY7s are a nice valve for more vintage-type brighter, lower-gain sounds - they were used as the original fitting in many 1950s Fenders - but be careful in reissue Fenders since Fender reduced the rating of the resistor which feeds it, and you can burn the resistor out.

12AV7s exist too, and are pin compatible with the others, but they're very rarely if ever used. I think I've got one somewhere, but I don't remember what it sounds like.

musicegbdf

Interesting ... the 12 AY7's sound like might be worth trying. I assume I replace all three and don't mix them..?

"Phase inverter isn't the usual design, but you can mess with it."
 I don't know what this means but sounds like I can change the valves without worrying... 

Sorry was not around last night a few domestic crisis ...

ICBM

musicegbdf said:

Interesting ... the 12 AY7's sound like might be worth trying. I assume I replace all three and don't mix them..?

No - it's not a good idea to do this. At best you won't find out which valve is doing what to the sound, at worst you might cause problems.

Replace *one* valve and see what the effect is - 12AY7s are best (in some amps only really suitable) for V1 positions. 12AT7s are generally better for phase inverters. It's quite possible you might like both substitutions, so it's worth buying one of each - but no more than that until you know what effect they have.

In this amp I wouldn't put a 12AY7 in the reverb drive position (almost certainly V2, hopefully Van_Hayden can confirm) without checking that it won't harm anything.

musicegbdf

Oh thks... will wait an see what Van-Hayden thinks as he seems to know my amp in detail...

Van_Hayden

It's reverb recovery and running low voltage so it'd be fine. Also the anodes should be 2w metal film types not 1/2w like fenders.

I can't remember what the phase inverter is but it's not a common long tail pair. Could be a paraphrase....Dave used a few different designs.

And yes - just get a couple of valves and try them one at a time.

musicegbdf

Sorry to keep asking questions ... So I should try them in V1 position ?

ICBM

musicegbdf said:

Sorry to keep asking questions ... So I should try them in V1 position ?

From what Van_Hayden is saying it should be safe to try them in all the positions. He knows the amp design and I don't, so take his word for it - the reason not to use 12AY7s in V2 in many amps is because V2 is sometimes a cathode follower, or if it's the reverb driver and has high (Fender-like) plate voltage. If it's neither a cathode follower or high voltage, all the valve types should be safe.

My guess would still be that a 12AY7 will sound best in V1 and a 12AT7 in V3 (phase inverter), but since the amp sounds like it may be different from 'conventional' design, even that's not a certainty. In the Mesa Blue Angel which is another non-conventional amp, a 12AY7 works best in V2, for example.

musicegbdf

Again tks looks like I should get one of each and try them in different positiond . From what V H has said sounds like I won't damage anything. Will try the AY in V1 and the AT in V3 first

Van_Hayden

Low plate voltage and there's no verb transformers to drive.

musicegbdf

Ordered from Karltone. 12AY7. 12AT7 and a new 12AX7. All EH.