Class 5 Output Transformer

[dido__15]

Hello,

I am new to this forum, and signed up because there seems to be more information about this amp here than the rest of the web combined. Kudos.

I'm hoping that SteveD will chime in on this one, as it is a technical question regarding the design of the amp.

My question is, what is the primary impedance of the output transformer, and is it dependent on speaker load? If possible could I have a spec sheet for the transformer? I realise these questions will probably get a sharp "no" but I have a couple of 8ohm speakers that I would like to try with this amp. If the primary impedance is dependent on speaker load (like most Hammond types) then plugging an 8ohm speaker into the 16ohm jack will double the primary impedance attached to the output tube, resulting in a much less dark and more open sound.

Also, are the 4 clipping diodes arranged as shunts to ground or in parallel to the circuit? Marshall's customer service department refused to provide me with a schematic and I have no time to reverse engineer the circuit, but I plan on removing these as soon as possible.

Many Thanks,

James

[BrotherJ]

Clipping diodes?

[dido__15]

Clipping diodes?

Circled in red in the attached picture.



Although actually at a second look it could be the set of 4 in the bottom left. I'll have a closer look later. The question still stands.

[ClubAndCountry]

I believe those are the rectifier diodes for the DC filament supply for the preamp valves. The others are the rectifier diodes for the HT. You'll soon find out if you remove them... the amp will stop working!

Changing the speaker impedance can't *not* change the reflected primary impedance, no matter what the type of transformer. Using an 8-ohm speaker instead of a 16 will *halve* the primary impedance, not double it. The question is whether it will cause trouble - which depends where on the load line the stock primary impedance has been chosen. My guess is that it's fairly optimal and that changing it will result in loss of power, headroom or tone (or all three). But low-powered single-ended power stages are not usually particularly impedance-sensitive, so it may be worth trying it anyway.

[dido__15]

I believe those are the rectifier diodes for the DC filament supply for the preamp valves. The others are the rectifier diodes for the HT. You'll soon find out if you remove them... the amp will stop working!

OK, I may have been mistaken, the single-ended amps I have worked on in the past have only ever had one quad of diode rectifiers, a Jack of all trades if you will. I always though that separate rectifiers were only required in push-pull configurations. i.e. as the current supply to the tubes in a single ended amp is constant it only needs to be rectified once (or something along those lines).

Changing the speaker impedance can't *not* change the reflected primary impedance, no matter what the type of transformer. Using an 8-ohm speaker instead of a 16 will *halve* the primary impedance, not double it. The question is whether it will cause trouble - which depends where on the load line the stock primary impedance has been chosen. My guess is that it's fairly optimal and that changing it will result in loss of power, headroom or tone (or all three). But low-powered single-ended power stages are not usually particularly impedance-sensitive, so it may be worth trying it anyway.

You are indeed correct it will halve the primary impedance, I was typing faster than I was thinking again. I will defiantely give it a go and post my findings in that case. I have yet to find a mass-production amp's output transformer with an *optimal* primary impedance.

[SteveD]

Welcome to the forum dido_15 and BrotherJ.

Those diodes circled in the picture are indeed the bridge rectifier which converts the AC filament supply to DC for the preamp valves to minimise any hum pick up. You can clearly see where the PCB trace is originating from that feeds them.

There is no diode clipping or any kind of solid state signal circuitry in the Class 5 whatsoever!

As C&C correctly states, the nominal primary impedance of the OPTX will be halved so the reflected primary impedance when used with an 8 ohm load will result in a 15 to 20% drop in output power as the EL84 will then dissipate more of the available power (at the nominal frequency). When the amp is not being played the EL84 dissipates it's full power potential of around 12W because nothing is being shared with the OPTX which is actually and obviously the hardest on the valve!

The 8 ohm speaker will have it's own sonic agenda as well as specific affect on the frequency response caused by the impedance shift.

Sorry no specs or schematics can be posted here.

[ClubAndCountry]

There's actually no difference in the rectifier arrangement between single-ended and push-pull amps - both normally use single rectifiers. The difference is in the output section which is fed from them.

Dual rectifiers are used only where either a single rectifier can't handle the current - which used to happen with valve rectifiers which have a much lower forward current rating, and still does in some high-powered modern valve amps that use valve rectifiers, the best example of course being the Mesa Dual Rectifier model - or where separate supplies are used for the preamp and power amp, which is very rare but again Mesa make one, the Blue Angel. The reason here is to give the preamp a solid-state rectfier and more headroom than the valve-rectified power stage, so the amp is guaranteed to overdrive the power section only - but it could also be used to avoid crosstalk and stability problems via the power supply, possibly. Some early Marshalls with solid-state rectifiers (only the Major, as far as I know) also used dual bridge rectifiers, again because the diodes in those days couldn't handle as much current as modern ones - but since modern diodes can handle more current than any guitar amp needs, no amp has been made like that for decades.

Most modern Marshalls use a DC filament supply for the first preamp valves - this cuts down on hum induced via the cathode, particularly with higher-gain amps, but also due to the poor quality of some modern preamp valves that seem more prone to this than the old ones. (And to failing in positions with high cathode-to-filament voltages, such as cathode follower stages and some phase inverters.)

[dido__15]

Many thanks for your comments. Clearly I have a great deal to learn still. As you can probably tell this is a different sort of circuit to those I have been exposed to in the past, and so your input is valuable to me.

Thanks again,

James

[ClubAndCountry]

Actually, if you can find the time, doing a little reverse engineering on this amp will probably teach you a lot about how they work - it's a fairly simple circuit. It's maybe not the ideal candidate for experimenting with since it's a PCB amp and these are harder to work on - especially to avoid doing damage to with repeated component changes. Some sort of old handwired amp - many of these aren't at all expensive, if you don't go for a 'name' brand - or a kit of some sort would probably be the best place to start.

Good luck and have fun!

[burnsy]

Re: ClubAndCountry's suggestion
I can thoroughly recommend the amp kits available at www.ampmaker.com. Barry supplies 3 or 4 top quality amp kits with top quality components and comprehensive easy-to-follow instructions.
I made his 18watter which is based on the Marshall 18w circuit, which itself was based on Charlie Watkins (WEM) Clubman circuit. It was good fun to build and I learned an awful lot about how these things work along the way. It sounds absolutely fantastic - just like amps used to sound! and is my amp of choice for recording. You can hear my one in use on the sound tracks at www.myspace.com/thejurassikaband.

[ClubAndCountry]

That's a great recommendation burnsy - I had vaguely heard of these kits but hadn't seen them before.

The SE-5A (great name, although if you're not a vintage aviation buff you may not know why!) looks absolutely perfect. I think I know what I want for Christmas now :-). Even for someone experienced, a kit like this is still an economical way of building an amp since the total component cost is probably not that far short of the kit price, if you were to buy them all individually at retail... and all the hard work of punching out the chassis (which is the most difficult part to do well, for a home builder) has been done for you.

[dido__15]

Actually despite my limited knowledge I have done some building before, I have built an ax84 kit, (P1 preamp and the octal poweramp from the single ended lead) and done a lot of modding on a pair of epiphone valve juniors, I just don't have the time to fully reverse-engineer this amp. If a schematic does become available at some point I will try a few things after I've studied it a little. It was just the dual rectifier configuration confused me as I hadn't seen it before.

I actually prefer working on PCB's, I have a halfway-done turretboard valve junior with a lot of burnt leads that I doubt I'll ever be able to fix due to my limited soldering skills.

-- Fri Dec 11, 2009 10:27 am --

Actually despite my limited knowledge I have done some building before, I have built an ax84 kit, (P1 preamp and the octal poweramp from the single ended lead) and done a lot of modding on a pair of epiphone valve juniors, I just don't have the time to fully reverse-engineer this amp. If a schematic does become available at some point I will try a few things after I've studied it a little. It was just the dual rectifier configuration confused me as I hadn't seen it before.

I actually prefer working on PCB's, I have a halfway-done turretboard valve junior with a lot of burnt leads that I doubt I'll ever be able to fix due to my limited soldering skills.

[BrotherJ]

Welcome to the forum dido_15 and BrotherJ.

Those diodes circled in the picture are indeed the bridge rectifier which converts the AC filament supply to DC for the preamp valves to minimise any hum pick up.

Cheers for the welcome Steve!
I didn't think the Class 5 would have used clipping diodes, but it's good to have you on here to confirm it's not the case.

I've got a Class 5 on order over here in Australia, theres a bit of a wait due to the popularity of this one Can't wait to get it!