Transformer upgrades for Blackstar HT5

Mr_Claw

Righto chaps?

I'm thinking of replacing the transformers in my Blackstar HT5. I know that Hammond's 125B output transformer is a replacement for the HT5's, but I don't seem to be able to find the specifications needed for working out which Power and/or Choke transformers I'd need. MercuryMagnetics makes some, but they're ludicrously expensive and their website doesn't give any specs for them - it just says "drop in replacement".

The schematic lurks, among other places, here: https://pdfslide.net/documents/blackstar-ht5-full-schematic.html ; (I can't read schematics though as I know nothing about electronics!)

Are there any amp/mod people either with experience of changing transformers in HT5 amps or otherwise would be able to advise me please?

Thanks as usual.

Chris

Funkfingers

Mr_Claw said:

Are there any amp/mod people either with experience of changing transformers in HT5 amps or otherwise would be able to advise me please?

Talk to the organ grinder. Paging @ecc83

relic245

Just curious to know what are the advantages of changing the transformer? 

Funkfingers

Agreed. Little point (theoretically) upgrading the OT if you intend to stick with the horrid stock MIC Celestion loudspeaker.

Mr_Claw

Funkfingers said:

Agreed. Little point (theoretically) upgrading the OT if you intend to stick with the horrid stock MIC Celestion loudspeaker.

Thanks for the reply. For ref I've changed the stock speaker to a Fane A60 (alnico); the stock power valve to a '50s Sylvania 12BH7a, the stock preamp valve to a '50s RCA 12AU7 and put an Effectrode Blackbird in the FX loop (loaded with 1 x Sylvania long black plate 12ax7; 2 x Genalex Gold Lion 12ax7s). Frankly it sounds pretty sweet for a Blackstar HT5!

Chris

relic245

Mr_Claw said:

 Frankly it sounds pretty sweet for a Blackstar HT5! 

I think they sound pretty good anyway. I used to put mine through a zilla 2x12 cab and the clean channel turn up full was a really nice sound. 

Mr_Claw

relic245 said:

Just curious to know what are the advantages of changing the transformer? 

I've heard they add headroom / clarity, and that transformers are often skimped on by budget-mid-level amp manufacturers who often use cheap, lightweight transformers. But then that might be total balls. I'm happy to defer to the experts!

ICBM

Unless you're going to change the spec to something quite different there is no point.

You're also taking a risk with a 12AU7 in the preamp, unless you've also uprated the plate resistors to 1W.

Modulus_Amps

What are you hoping to get by replacing the transformers exactly, do you want more clarity, punch, more vintage smear and grind. You kind of need to know what you are aiming for to get there.

I don't know what the quality of the trannies are in the HT5, but you might change it for something and loose something in the feel of the amp that you like.

impmann

If you know nothing about electronics, I'd advise *VERY STRONGLY* against playing around with changing transformers. 

Also - when you changed the output valve, how did you bias it?

Mr_Claw

relic245 said:

Mr_Claw said:

 Frankly it sounds pretty sweet for a Blackstar HT5! 

I think they sound pretty good anyway. I used to put mine through a zilla 2x12 cab and the clean channel turn up full was a really nice sound. 

Yeah, it's not a bad amp. I'd love to play it louder, but I live in a flat and my neighbours would kill me. Thinking of getting an attenuator so I can push it a bit harder at domestic levels.

Stock, I thought the clean channel sounded fine but just a little bland, and the gain stage was waaay too nasal-metal for my liking.

With the changes? Well, it's never going sound like a vintage Fender (because it's not one!), but it certainly sounds like some variety of vintage amp now!

Sometimes cheap modular manufacturing can work in your favour: the entire pre- and power-amp section of the HT5 fits onto a removable board. The Fane A60 is a big ol' alnico speaker with a magnet that's waaaay too deep to fit most 12" combo cabs, but I just took the whole amp section off and put it in a box on top! 

Mr_Claw

impmann said:

If you know nothing about electronics, I'd advise *VERY STRONGLY* against playing around with changing transformers. 

Also - when you changed the output valve, how did you bias it?

Well I won't be doing *anything* - I'll get a professional to do it. I value my life too much to be messing with high voltage.

When I changed the power tube I took it JPF Amps on Denmark St to bias it (good job too as he found that the biasing was broken and fixed it!).

Mr_Claw

ICBM said:

Unless you're going to change the spec to something quite different there is no point.

You're also taking a risk with a 12AU7 in the preamp, unless you've also uprated the plate resistors to 1W.

Thanks for the comment. TBH I'm not sure about this - I had read on the Blackstar forum that this was fine and that plenty of others had done this as the preamp section was designed to accommodate any of the 12a*7 family.

That said, you've now made me paranoid that something terrible will happen. To be on the safe side, I have other suitable drop-in valves that will work (Tung Sol 5751 / Sylvania 12ay7).

Thanks,

C

ICBM

Mr_Claw said:

Thanks for the comment. TBH I'm not sure about this - I had read on the Blackstar forum that this was fine and that plenty of others had done this as the preamp section was designed to accommodate any of the 12a*7 family. 

That said, you've now made me paranoid that something terrible will happen. To be on the safe side, I have other suitable drop-in valves that will work (Tung Sol 5751 / Sylvania 12ay7).

The 5751 is safe, the 12AY7 is not.

They are interchangeable physically, but the amp is not intentionally designed to accommodate the valve types other than the 12AX7. The problem is that as the gain of the valve is reduced, the current through it increases - which increases the plate resistor dissipation, to above the resistor's power rating if you go too far. A 12AT7 is just on the limit, a 12AY7 over it, and a 12AU7 well over it.

The only terrible thing that may happen is that one of the plate resistors could fail - to be honest I'm a bit surprised it hasn't failed already - but they are at least some of the few components in the amp that are fairly easy to change, since they're traditional through-hole parts rather than surface-mount. If one does fail, fit a 1W replacement and it shouldn't do it again.