EL-84 tubes?

[nashvillebill]

Nope.

Major problem with the 50C5 amp....it likely is a "hot chassis". Meaning there's no power transformer, and the line voltage is powering all the filaments. Major death trap potential there.

Second problem: you've got a single-ended output transformer. For push-pull you need a different ouput transformer.

So, to convert this amp to p-p EL-84's you would need a) a new power transformer--and not just to solve the hot chassis problem, but also to provide the 6 volt filament supply for the EL-84's as well as proide the proper B+ (high voltage) for the EL-84's. And you would also need b) a newoutput transformer for p-p and with the proper impdance ration. And you would also need c) a new preamp section because by doing away with the line voltage heater filament of the 50C5, you no longer have a heater filament supply for the preamp tubes. And d) likely the rectifier is a 35-volt heater supply, so it needs to go as well.

Yikes!

 

[nashvillebill]

...After thinking a few minutes, perhaps I misinterpreted the 50C5 question a little. Is this a conversion project, or rather a general question of "Can I make a P-P amp using tube "A" sound like my SE amp that uses tube "B"?"

In which case, it's my opinion the choice of tube--A,B or Q--doesn't matter a whole lot. It's the topology of the whole amp that matters. In other words, take the single-ended amp with the 50C5, stick a separate filament supply in there for 6 volts, rewire the socket, and drop a 6V6 in there, IMHO the amp probably will sound pretty darn near the same. The "character" of these little amps comes not from the quirky tubes (they bought large quantities of no-longer-needed TV tubes oftentimes) but from their very design. A single ended, cathode-bias design on the power amp side. Grid leak bias on the front end. Minimal parts, meaning no grid-stopper resistors (hence the lovely "farting out" at higher volumes). Undersized output transformers, of dubious audio quality.

So going to push-pull changes the very "soul" of the amp. Sure, you *could* go to cathode bias on the p-p, but it's still no longer single-ended. And you still have a phase splitter of some sort to add to the circuit.

The amp then becomes...say a Fender Deluxe instead of a Fender Champ. That's allright, though, I'm not crazy about Champs anyways.

 

[pie_man_25]

...After thinking a few minutes, perhaps I misinterpreted the 50C5 question a little. Is this a conversion project, or rather a general question of "Can I make a P-P amp using tube "A" sound like my SE amp that uses tube "B"?"

In which case, it's my opinion the choice of tube--A,B or Q--doesn't matter a whole lot. It's the topology of the whole amp that matters. In other words, take the single-ended amp with the 50C5, stick a separate filament supply in there for 6 volts, rewire the socket, and drop a 6V6 in there, IMHO the amp probably will sound pretty darn near the same. The "character" of these little amps comes not from the quirky tubes (they bought large quantities of no-longer-needed TV tubes oftentimes) but from their very design. A single ended, cathode-bias design on the power amp side. Grid leak bias on the front end. Minimal parts, meaning no grid-stopper resistors (hence the lovely "farting out" at higher volumes). Undersized output transformers, of dubious audio quality.

So going to push-pull changes the very "soul" of the amp. Sure, you *could* go to cathode bias on the p-p, but it's still no longer single-ended. And you still have a phase splitter of some sort to add to the circuit.

The amp then becomes...say a Fender Deluxe instead of a Fender Champ. That's allright, though, I'm not crazy about Champs anyways.

it's basically a "can I make a p-p amp that sounds like this SE amp"

I honestly hadn't thought about phase-inversion just yet, I've just been looking at schematics for this amp, as well as a head that was produced by the same company - which was p-p and had two 6v6 tubes for the power amp. I just dig the vibe that this amp has, and want some similar-sounding head/combo with a bit more juice that I could use for some acoustic gigs I've been playing. Are you suggesting that I replace the 50C5 with a 6V6, or should I run them in parallel SE mode? Not to mention, it's tube biased, with another old radio tube, I'm not sure if that complicates things or not, but it's a 35W4.

I'm well-aware that this thing is somewhat of a death-trap with its grounding scheme and lack of power transformer, so I never play it out, I've also been thinking about an isolation transformer. I'm just not sure about modding this amp in general, because even though it's worth prettymuch nothing, it's still kindof cool and I don't want to mess with its mojo.

All this being said, I think I'll take my queries to SEwatt or something similar.

 

[walterw]

(to the OP)

sounds like you have a neat little low-powered tube guitar amp here.

there's really no point in trying to make a bass amp out of it, you'd have to change everything on it!

it's like trying to convert an old VW beetle into a towtruck.

just plug it into an actual bass cab (at the correct impedance, please!) for quiet at-home bass noodling.

 

[nashvillebill]

it's basically a "can I make a p-p amp that sounds like this SE amp"

I honestly hadn't thought about phase-inversion just yet, I've just been looking at schematics for this amp, as well as a head that was produced by the same company - which was p-p and had two 6v6 tubes for the power amp. I just dig the vibe that this amp has, and want some similar-sounding head/combo with a bit more juice that I could use for some acoustic gigs I've been playing. Are you suggesting that I replace the 50C5 with a 6V6, or should I run them in parallel SE mode? Not to mention, it's tube biased, with another old radio tube, I'm not sure if that complicates things or not, but it's a 35W4.

I'm well-aware that this thing is somewhat of a death-trap with its grounding scheme and lack of power transformer, so I never play it out, I've also been thinking about an isolation transformer. I'm just not sure about modding this amp in general, because even though it's worth prettymuch nothing, it's still kindof cool and I don't want to mess with its mojo.

All this being said, I think I'll take my queries to SEwatt or something similar.

No, I'm NOT suggesting that you swap a 6V6 for the 50C5, I was merely trying to point out that the topology of the amp has far more of an influence on the "sound" than the particular tube used.

The 35W4 is a rectifier tube. This provide the B+ (which is the high-voltage for the amp). The output tube (the 50C5) is biased by cathode bias, not by another tube.

Try AX84.com for DIY guitar amps. Also some books such as Jack Darr's "Electric Guitar Amplifier Handbook" for some basic discussion of amp design. Merlin Blencowe has two good books "Designing Tube Preamps for Guitar and Bass", and "Desiging Power Supplies for Tube Amplifiers"; his site http://www.valvewizard.co.uk/ also has a tone of useful info to help get you started.

 

[AstroSonic]

well...this saga has taken an interesting turn.

took the amp back to GC, told them of my concerns and offered them 100.00

the guy looked at it and said- no wonder the reverb doesn't work...you don't need a new tube, someone took out the reverb tank!

sooooo....they sold me the amp for 110.00...sweet! my son has a holy grail reverb, so that is no problem.

took it by my amp doctor, and he will be out until next wednesday.

get a new tube set (minus the reverb tube...), and have him go over it.

i anticipate we will be up and running for a total 150.00 bucks or so, counting the purchase price of 110.00. (hell, we are already up and running, just needs some tuning.)

Glad to hear things have worked out. One other thing you may want done (in addition to new tubes) is to have the amp recapped. Your amp tech will probably bring this up. If (when) he does, you should consider it good advice. It will add a little more cost to your project, but will ensure many years of safe and dependable use.

Good luck!

 

[MAMMOTHvolume]

I'm going OT here, but do you know anything about the 50C5 power tube? I've got this PEPCO made single-end amp with one, I've been wondering if I could make a head-version of it that's got a push-pull power section using current production EL-84s instead of the 50C5 and still have it sound reasonably similar.

I've got an amp that uses a pair of them. They're not very similar soundwise to el84's. I'd go the 6bq5 / 6v6 route for a similar sound and keep it single ended. Single ended has a very different sound of it's own aswell!