To recap or not to recap?

[horn man]

Hello all. I need some advice on what to do with an older amp, a Premier Multivox (I don't know the model, but the schem is listed on schematicheaven.com under Bargin Bin Amps, Premier-Multivox, model guitar? 1967). The amp sounds pretty good but from what I gather, should be putting out a healthy 11 watts or so from the 7591 power tube. I know its no where near that since it can be turned all the way up in my den without making anyone's ears bleed. (for comparison, an 18wt lite rattles the windows in the same room). My son's guitar teacher has the amp now and doesn't think the amp needs a cap job, maybe just a new power tube (its got the original Multivox branded power tube). He doesn't think its excessively noisey/humming at all. Further more, he thinks it has about as much volume as a Fender Champ from the same era. Now I have a lot of respect for him, as he is a very experienced player and has owned every guitar/amp rig imaginable through the years. But, I've heard modern 5wt amps, like the Epi Jr. head, and it is painfully loud in a small room when dimed. And, all the forums recommend replacing the caps on a 40 year old amp. So what's everyone's opinion? Replace the power supply electrolytic caps and the power tube, or am I expecting too much out of the amp?

[Alexo]

I'd check the voltages and see if everything's working properly. If you have a signal generator, you can measure the output power with an RMS AC volt meter and see what it's actually putting out.

[tweedeluxe]

If the filter caps are original, they should be replaced ASAP. If it has the original 2 prong power cord, you will also want to remove the death cap(s) and install a 3-prong cord.

Try plugging the Multivox into one of the "louder" amps' speakers - the stock speaker is probably not very sensitive.

Also, replace the 100k resistor hanging off the input with a 1meg or higher. You could also change the first stage to cathode bias quite easily.

[CaseyJones]

Oddly enough I have one of those in my permanent collection. Then again, maybe it ain't so odd considering the breadth and diversity of my collection.

Some call it, "junk"...

It probably isn't a fair comparison with a Valve Junior, they're different circuits. The Premier uses a grid leak biased 12AX7, it uses a different P.A. tube and did you run both amps into the same speaker?

On the cap job... it wouldn't hurt. I don't know about yours but mine still has the original Mallory multi-section can. There's plenty of room inside the chassis to just fly the few caps off terminal strips. Hint: Buy the parts from Apex Junior, do it yourself and spend more money for shipping than you did for parts!

Just for chuckles you could re-wire the amp to take a 6V6 tube. The OT needs to be changed for a 6V6, it's around 3k into 8 ohms in mine which gives me a choice of changing the OT or the speaker. It just happens that I get 6V6s and OTs for free or close to it. 7591s belong in old Ampegs IMHO so that's where mine go.

[Firestorm]

I know the specs say 11 watts, but with 19 watts max dissipation, 11W power out is overly generous. You'll also never get there unless the preamp really drives the heck out of the 7591. This one looks a bit light -- the mu-amp front end should be fine (but not very clean), but the second stage doesn't seem to have much gain in the design.

For sure, check the 7591 and the 12AX7; the amp will work better if they're strong. Check the speaker too. As to the caps, a single-ended amp will hum if the main filter is bad (actually it will usually hum even if the cap is good), but if another section is beginning to leak, it can take out the dropping resistors, so check the 150R and the 4K7, too.

I'd replace the cap on general principles. You should be able to get that one (or close) from AES.

[horn man]

Thanks for the advice, all. The last time I looked inside the chassis, every thing looked pretty much original, including the cap can. All the tubes are branded "Multivox" except the preamp, which is a Sylvania. I'll try hooking the amp up to another speaker (yes, the 18wt is using a G12H at 100db sensitivity; I believe the Multivox has its original, alnico? 12"). Like I say, it has a nice classic old sound, so I don't want to go to the trouble or expense of changing trannies or rewiring for other tubes. But I will change out the AC cord, replace the PS caps, and mabe the power tube and tremolo tube. I see that JJ Telsa has a new production 7591 for around $15.00 or so. BTW, do temolo tubes commonly go bad? The tremolo on this amp is a little intermitant. Maybe the sockets need cleaning or there is a bad solder joint somewhere. Antique Radio Supply has the 6AV6 for about $3.00 (NOS ).

[horn man]

Thanks for the advice, all. The last time I looked inside the chassis, every thing looked pretty much original, including the cap can. All the tubes are branded "Multivox" except the preamp, which is a Sylvania. I'll try hooking the amp up to another speaker (yes, the 18wt is using a G12H at 100db sensitivity; I believe the Multivox has its original, alnico? 12"). Like I say, it has a nice classic old sound, so I don't want to go to the trouble or expense of changing trannies or rewiring for other tubes. But I will change out the AC cord, replace the PS caps, and mabe the power tube and tremolo tube. I noticed JJ produces the 7591 for around $15.00 or so. BTW, do temolo tubes commonly go bad? The tremolo on this amp is a little intermitant. Maybe the socket need cleaning or there is a bad solder joint somewhere.

[CaseyJones]

Yours has trem? Mine doesn't. There's a hole in the chassis where a 6AU6 is supposed to go but no 6AU6. There's some kind of logo (orig) glued over the hole on the front where the trem pots would be.

The 6AU6 is a little oscillator in this amp. That particular trem circuit is twitchy, sometimes it oscillates, sometimes it doesn't.

Knowing Multivox the stock speaker probably crap. I wouldn't know, mine is long gone. There's an unwritten rule, "Amps found in dumpsters don't come with speakers!"

[horn man]

I got this one from the "ebay dumpster", many times the same thing. It supposedly came from a retired tech's shop. He probably played it only on Sunday afternoons, with a tiny nip of sherry, etc... Surprising, it has an all plywood cabinet (albeit, thin) inclucing the baffle and rear panel. True point-to-point inside with a couple of terminal strips. I guess no need for premium caps for the recap, huh?