Discuss the stuff that makes your ears bleed.

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#801844
Hi guys,

I'm trying to help a friend of mine fix his Holden 50 combo, but I'm a bit of a noob when it comes to amps and tubes.

He says he had the amp serviced and it worked perfectly, but accidentally knocked out one of the tubes and it broke.

Questions:
1. Are these the right tubes? I have an ECC1, a 12AX7, and one that doesn't have any markings. There should be a fourth tube I think, but it broke.
2. What's the fourth tube that I need?
3. What order am I supposed to put them in?

I think if we just pop everything back in it should work... Any thoughts? Pics below. :)

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#801849
It’s probably just 4 x preamp tubes you need - the ECC81 is just a slightly lower gain version of the 12AX7. One more of either of those should do the trick; the most important one from what I understand is V1 (the one that sits nearest the guitar input).
#801866
Mini Forklift Ⓥ wrote:It’s probably just 4 x preamp tubes you need - the ECC81 is just a slightly lower gain version of the 12AX7. One more of either of those should do the trick; the most important one from what I understand is V1 (the one that sits nearest the guitar input).


Sounds easy enough, thanks for that info. :)

Kris wrote:i have a photo of my old holden with guts out,ill see if i can dredge it up


That'd be awesome. Is it a Holden 50 as well?
#801885
Type ‘schematic holden 50w amp’ into Google, there seems to be a circuit diagram available, but I can’t share it directly—google thinks it might be under copyright

But yeah, it looks like all 4 pre-amp tubes are ecc81 or 12ax7
#801889
olegmcnoleg wrote:Type ‘schematic holden 50w amp’ into Google, there seems to be a circuit diagram available, but I can’t share it directly—google thinks it might be under copyright

But yeah, it looks like all 4 pre-amp tubes are ecc81 or 12ax7


Cheers. So that one with no label must be a similar variant.

So if we went with an ecc81 for the fourth tube, that'd suit a cleaner tone right?
#801906
I think the ECC81 is the Phase inverter and is nearest the output valves.
The others are all ECC83/12AX7
#801915
What Alan said.

To say the ecc81 is a lower gain version of the 12ax7 isn't quite right. It does have a lower mu, but there are also some other differences.

A 5751 is a lower gain version of a 12ax7.

If the broken tube was a 12ax7 and you want a cleaner tone, you should get a 5751 to replace it and put it in V1
#801926
Here's a couple photos of one I worked on in the past - the serial # is pretty close to yours. The tube chart on the back calls for 4x 12AX7/ECC83, so your ECC81/12AT7 may have been a mod someone tried for less gain - probably in the 4th position as the phase inverter. You can really use whatever combo of 12A_7 preamp tubes sounds best, although V3(?) might need to be a 12AX7 for the tremolo to work.

The KT88 series of Holdens (Wasps) had a 12AU7 as a cathode follower buffer stage between the phase inverter and power tubes - a pretty cool configuration. That's a case where a 12AU7 is not really interchangeable with a 12AX7, etc. (your EL34-based Holden 50 doesn't use this circuit, though).

https://imgur.com/a/0DA6mGn
#801929
a 12AY7 or 12AU7 sound better than a 12AT7 at V1 imo.

12at7 in V1 can sound brittle, the Holden is a low gain amp anyway so switching out the 12ax7 to a 12AT7 might make the amp sound sterile.