Rectifier substitution

bandmaster188

Was just reading some stuff on putting different rectifiers in amps and some of what I read seemed contradictory. One of my bandmasters runs on a 5u4 and the other a gz34. Is it safe to try a 5y3 in there? One bit of info says it's safe and it will give you earlier breakup and more sag and another says 5y3 won't last very long in a 6l6 amp. What s the wisdom on this?

ICBM

The second.

The current draw of a Bandmaster is too high for a 5Y3 to work reliably, even with the reduced power it will put out due to the greater voltage drop. It will be pushing it on the AC voltage rating as well.

There are three things you need to consider: current rating, voltage rating and filament current draw. You must not exceed any of these ratings for the valve when substituting. (Filament current draw matters to the amp not the valve, so should not be increased.)

For the common types:

5Y3 - 125mA, 350V, 2A
5U4 - 250mA, 450V, 3A (there are slight differences between the G, GA and GB but not enough to matter for this)
GZ34 - 250mA, 450V, 2A

So you can see it's always safe to substitute a GZ34 for either of the other two types, but in most cases not the other way round. The GZ34 has the lowest forward voltage drop and so the tightest sound. It will increase the HT voltage slightly too, so be sure the rest of the amp will be OK with that.

The 5V4 is a possible substitution - the ratings are 175mA, 375V, 2A. It's right on the limit but a pair of them run safely in my Trem-o-verb, so should just about be OK in the Bandmaster. It will give a slightly tighter sound than a 5U4 though, which is probably not what you want.

bandmaster188

Yeh, thought it was gonna be the latter. I had a 5y3 kicking around and thought I'd try it out but I don't think I'll bother. Cheers for the info ICBM!

ecc83

Might I just add that different rectifier valves have a different maximum capacitor value they can feed?

This also varies with other factors such as AC input to the anodes but for instance it is 64microfarads max (iirc) for the GZ34 which in practice means 47mfd(50mfd) .

FYI I have found the TAD GZ34 to be very rugged.

Hardly worth the mention but if you come across a very, very, very early A15 or A30 this needs to be checked and a mod done if a 100mfd is found as the res' cap.

Dave.

bandmaster188

I'll have look at those TAD gz34's. I could do with a spare, I'm still using the sovtek one you kindly sent me after I blew my mullard up!

ecc83

bandmaster188 said:

I'll have look at those TAD gz34's. I could do with a spare, I'm still using the sovtek one you kindly sent me after I blew my mullard up!

PM or email me.

Dave.

bandmaster188

Pm'd Dave