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Grommes Hi-Fidelity

ozmoid

ozmoid

Lunatic Member
Picked a "Pig-in-a-Poke" old cabinet speaker for cheap to see what kind of driver was in it, and it came with a Garrad TT (50's era stamped steel and bakelite!) and this Grommes Tube Amp 56PG.
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This is apparently the original "multi-media" amp - no end of input choices. (It is a mono amp, BTW.)
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Surprised to see that it has been recapped - not a paper cap in sight! Am I right, or could these be the originals? Just look too new to match the amp to me.
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Of course, the first thing I did was tear the back off the speaker cab to see what I had - it's an Electro-Voice SP12B. Sounds BIG running off the tube amp - I piped a little Cab Calloway through it, thought that would be appropriate for it's maiden run. Cab is very forward and "live" in front of the band, lots of mid in this speaker, rolling off more at the top, plenty of bottom on these!
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The Amp controls need a round of deoxit, but otherwise the set sounds GREAT!
I feel like I did OK at $36.00 for the whole haul. :thmbsp:
 

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Had the bottom off again to deoxit the controls, and there are a few older caps left, mostly small values. Controls are being a bear - after I cleaned the first time, they are scratchier than before. I'm gonna give them a couple more rounds before I start tearing out the controls. Any suggestions? I'm giving at least 50 "spins" and a generous squirt of DeOxit.
 
ozmoid said:
This is apparently the original "multi-media" amp - no end of input choices.
Click to expand...

You can actually match a bucket of water to one of the inputs!

:D

$36 for the EV alone would have been fine. That is a great "full-range" speaker. I had a set of them in big "Console-Art" cabinets and added a set of Jensen RP-104 horns to them. The horns were just layed on the top of the cabs with a cap in series to highpass them. The combo sounded great. I ended up moving everything in a space crunch but always liked the result enough to think about duplicating it again sometime. Your cab style looks real familiar. Do they have a maker's tag?

Enjoy!
 
Great looking find.....That's a good question on the pots. I would like to hear the answer as well. I do not see why it wouldn't be the same as a SS amp, but I have never tried to do tube amp pots....
Bump
 
It might not hurt to throw a little Caig slider lube in there too... I have some at the house. I think when the grunge is loosened (on older pots especially) it floats around in the pot and makes for circuit interruptions (read static). The worst of my potentiometers have gotten lots better after cleaning then lubing them. Clean looking little amp, and great finds!

BTW - You now offcially suck, cause I am now officially jealous!
 
soundmotor said:
<SNIP>That is a great "full-range" speaker. I had a set of them in big "Console-Art" cabinets and added a set of Jensen RP-104 horns to them. The horns were just lying on the top of the cabs with a cap in series to highpass them. The combo sounded great. I ended up moving everything in a space crunch but always liked the result enough to think about duplicating it again sometime. Your cab style looks real familiar. Do they have a maker's tag?

Enjoy!
Click to expand...
Yes, I am really surprised - a single speaker has no business sounding this good! The cabs are by Argos. After doing a little research on AK, I'll be looking for a EV TW-35 tweeter and crossover to match this speaker. The cabinet already has a 1-3/4" x 4" cut-out! I thought that was an odd shape for a port :scratch2: but it is obviously for the tweeter. The SP12B has a rating down to 30hz for the bass, but rolls off on top at 13,000 according to the EV lit. (Thanks to mhardy for the link!) The top-end boost the tweeter will add should make this an awesome little mono unit! Due to the pot corrosion (I assume) the speaker came to me hooked up to the 8 ohm output, but EV lists this as a 16 ohm speaker. The response range has improved since I moved the speaker wires to the correct screws on the amp.

cableguy said:
Great looking find.....That's a good question on the pots. I would like to hear the answer as well. I do not see why it wouldn't be the same as a SS amp, but I have never tried to do tube amp pots....
Bump
Click to expand...
The pots really ought to be clean by now - I have done 6 rounds of DeOxit and over a thousand "twists", putting pressure on them different directions and every other trick I could find on AK or think up. I've taken one loose enough to see that breaking it open and reassembling it will not be a fun project. I have emailed Grommes for any info they may have about the values of this control so I can find a replacement. Grommes is currently re-releasing some of their old tube equipment, maybe they will have a suitable replacement for me!

I do have the pots partially working, if I had never moved them the first time all would probably be fine. There are about 4 or 5 points of good contact on the Bass and Loudness controls; the Volume, Treble, and input selector work OK. A tiny amount of static on the Volume, but the Bass and Loudness are all but unusable.

fotno said:
It might not hurt to throw a little Caig slider lube in there too... I have some at the house. I think when the grunge is loosened (on older pots especially) it floats around in the pot and makes for circuit interruptions (read static). The worst of my potentiometers have gotten lots better after cleaning then lubing them. Clean looking little amp, and great finds!

BTW - You now offcially suck, cause I am now officially jealous!
Click to expand...
So when can I expect to see you with lube in hand? :music:
 
Too bad that Utah horn tweeter went out with the last load of scrap aluminum... it was about that size.

Sams index lists this amp in folder 335-7 - I don't have that one. I have the one for the 55C, which looks almost identical...
 
Tom Bavis said:
Too bad that Utah horn tweeter went out with the last load of scrap aluminum... it was about that size.

Sams index lists this amp in folder 335-7 - I don't have that one. I have the one for the 55C, which looks almost identical...
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The Grommes website has an old catalog up, but this amp is not listed. All the others in this style and No. range are 10 WPC, 20 peak. I'm assuming that's about where this one lands too. I will hopefully find out more when I hear back from their tech support about the pots - Since it's the weekend, I'm not holding my breath for a reply just yet!
 
ozmoid said:
Surprised to see that it has been recapped - not a paper cap in sight! Am I right, or could these be the originals? Just look too new to match the amp to me.
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:thmbsp:
Click to expand...

That striped cap near the upper left of the picture (looks like 0.047uf) (color code 473, like resistors it means 47000pf) should be replaced. It's just a paper cap inside a plastic shell, and are very prone to being bad.

Also, the output transformer (bottom left) looks like a replacement.
 
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wa2ise said:
That striped cap near the upper left of the picture (looks like 0.047uf) (color code 473, like resistors it means 47000pf) should be replaced. It's just a paper cap inside a plastic shell, and are very prone to being bad.

Also, the output transformer (bottom left) looks like a replacement.
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Yes, the transformer does not cosmetically match the other, and all the other Grommes photos I can find show a matching pair. And yes, there are a number of caps that have not been replaced. I was so excited to see the newer caps when I opened it up (less work for me!) that I completely blocked out the ones that still need replacing.
 
OK, finally got around to tearing down one of the bad pots to see what's up.

A little knowledge goes a long way - towards destruction. These are bakelite or resin discs with a baked-on graphite coating for conductivity. Apparently the graphite was flaking, and all the DeOxit and twisting just stripped it off faster. Is there any way to "resurface" these?

I have a Sam's orderd for this amp, hopefully I will be able to find modern replacements. Right now I can't even get a reading to see what the values WERE, the contact surface is missing over too large of an area, and no longer completes a circuit. The one I'm most concerned with replacing is the Loudness control - it has 5 taps.
 
Tom Bavis said:
Too bad that Utah horn tweeter went out with the last load of scrap aluminum... it was about that size.

Sams index lists this amp in folder 335-7 - I don't have that one. I have the one for the 55C, which looks almost identical...
Click to expand...
Thanks for the Sam's info Tom, I have located the photofact.

On the .047 capacitor (confirmed by the Sam's), what should I use to replace the paper cap? Electrolytic or film/foil?
 
Thanks, blue_lateral! You're welcome to come by when I get it running again :naughty: . Seriously if you are ever on this coast, look me up!

I am ordering new pots since the old ones were pretty dead already, and then I finished them off with DeOxit. I'm short about .40 of meeting the minimum order, that's why I need to know about the cap - it will bump me over the limit!
 
ozmoid said:
On the .047 capacitor (confirmed by the Sam's), what should I use to replace the paper cap? Electrolytic or film/foil?
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I have used .047 orange drops and illinois capacitors to replace those black beauties. They both have a nice sound to them.

You don't want an electrolytic cap. I don't think they make that small anyway.
 
It's kind of hard to tell from the pictures, but don't I see two more of those black beauties tucked up by the pots?

Oh, and thanks for the invitation, :D If I ever get down that way, I'll definitely look you up.

John
 
OK, ordered the replacement pots and caps from Ocean State today. The pots were physically the closest to the originals of any I could find. I'll also be building a folded-horn cabinet for the EV driver (similar to jay's speaker project found HERE ), and I'll cross it over to a "three-way" University driver I picked up. (Thanks to Paul C for the advice on the speaker box!). The university driver has a built-in crossover, you can see the speaker HERE. It is an 8" cone with a center-mounted horn, I think it will couple nicely with the EV. :music:

I'll post pics as the set-up progresses.
 
This is gonna be a cool thread to follow. I really want to do a mono set-up some day. As goofy as it sounds, I think stereo can be distracting, at least for me. I find myself concentrating on the seperation more than the actual music at times. :headscrat The other thing about mono that is appealling for me, is the fact that you can take advantage of really cool stuff that you only find one of (drivers, tubes, etc.). Good luck with the project and keep us well posted (pics). Jay
 
Hopefully, Oz and I will be able to do a co-op type project on the Grommes Jay, and if I know OZ there'll be lots of pictures. Good thing about living so close is that we can share tools, materials and experience, and it makes the whole thing a lot more fun. I've never soldered a pot into an amp before, just guitars, so it should be fun learning some new things.
 
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