Strange tube in Gibson GS100 Tube bass head

[dude]

Hello all,

I fixing a yard-sale special, Gibson GS100 bass head from early sixties. It has a blown power resistor and I'm going over everything, valve 8 is a strange little tube I've never seem before. Looks like it reduces the rail voltage, it broke in half in my hand. Any suggestions what might be a good replacement? Seems to reduce the voltage about from 425v to 350v. It's toast.

al    

[sluckey]

It's a 0C2 voltage regulator tube. Think 105v zener. They're still available and cheap.

[dude]

Thanks, Sluckey I appreciate your help. I figured it wasn't available. It's a strange little thing. Who might have it?

al

[John]

http://cgi.ebay.com/0C2-Tube-New-Raytheon-Voltage-Regulator-Tubes-/120364806987#ht_500wt_1081

That guy does. :)

http://www.tubesandmore.com/scripts/foxweb.dll/moreinfo@d:/dfs/elevclients/cemirror/ELEVATOR.FXP?item=T-0C2  And so does AES. Hope this helps!

[dude]

Thanks,

I see the tube on AES but the old OC2 has ho socket, it's just hanging in a rat's nest, see pic's. Rather than have to drill and put in a socket any other component that would reduce the voltage 75v safely, a Zener would get pretty hot I would think?

al

 

[sluckey]

but the old OC2 has ho socket, it's just hanging in a rat's nest, see pic's

Do you think that red thing is a 0C2 tube??? It ain't. A GA-100 chassis has the OC2 mounted in a socket at one end of the chassis right next to the rectifier tube. Look at the schematic you posted. I don't think your amp matches the GA-100 schematic you linked to. Could you post a pic of the entire top of the chassis so we can see the actual tube placement?

[dude]

OK, sorry for my ignorance, I'm not too good with a build without a board, AKA rats nest.

Something's fishy here. The amp is a head with a metal case, pictures of the case say it's a Gibson GS100 as you can see.

According to the schematic there are 8 tubes, as you can see this amp has two 6Eu7's, two 7591's ?, and a 5AR4. that's it.

It's not the GS100 schematic I posted above, but it sure looks like it's original something from that era, you can see the old early sixties filters leaking and original wiring from the 60's.  

Is there another GS100 that uses two 7591's, two 6EU7's and a 5AR4? I see no OC2. That du-hickey, hanging off the fixed bias pot, what the heck is it.

Now I don't know what I have here. The 7591's have a different pin-out then the 6l6s butthe wiring on the sockets looks original.

Can you look the gut shots and tell me what I have? I've searched the GS100 schematic and this amp is not that, there are no open tube hole sockets either telling me it's been modified years ago???  

I'm saying GS, I meant GA100.

Now I'm thinking that maybe someone put this chassis in the GA100 case...? But it sure looks like this chassis was original to the case as it has fifteen little tin screws all fitting nicely in the chassis holes???

[jjasilli]

Free your regulator tube and let it glow
Let it blossom
Let it flow

[sluckey]

pictures of the case say it's a Gibson GS100 as you can see.

I see GA 100 on the front, not GS100. I'm thinking the GS series amps were solid state (or was that GSS series?

According to the schematic there are 8 tubes, as you can see this amp has two 6Eu7's, two 7591's ?, and a 5AR4. that's it.

According to the GA-100 schematic there are 9 tubes. Your amp is clearly different. Unfortunately that's very typical of Gibson.

That du-hickey, hanging off the fixed bias pot, what the heck is it.

I'm thinking light bulb/photocell (early optocoupler), similar to what you may find in a tremolo circuit. Without a schematic, I'd guess it's probably used in a compression circuit for that amp??? I'd probably wrap it with tape and concentrate on the burnt resistor.

Forget everything I said about 0C2. It's not applicable to this amp. Your amp looks simple enough to draw a schematic.

EDIT... Forgot to mention. Your amp is from the Crestline series. Documentation is lacking but if you're willing to sift thru a google search you may get lucky.

[dude]

Thanks,

No luck with Google, all I found was a Youtube of that amp with the 7591's. Just showed the exact head and it being played. The guy did write saying it had the same tube layout as mine. Everything else is the GA100, 6L6 amp with 9 tubes.

1961 Gibson GA-100 Bass Amp

So, Gibson probably just put this circuit in a case that says AS100.....?

I'm not that knowledgeable to figure the value for that blown power resistor. Do you know of any other amp with the same tube layout that I can check out, that would probably help me.

5AR4, 7591, 7591, and two 6EU7s. Looks like this amp is fixed bias too and that red thing (ha, I keep thinking of my dog) I'll just tape it. where it separated, I can see it's some kind of senor against smooth glass with wiggley lines embedded in it...?

al

Thanks,

al   

[sluckey]

So, Gibson probably just put this circuit in a case that says AS100.....?

Could be. Gibson did a lot of stuff without documenting it (or they kept it in house). HotBluePlates may have some info.

I'm not that knowledgeable to figure the value for that blown power resistor.

Looks like a screen dropping resistor. I'd stick a 5K in it for testing. Be cautious of the filter caps. That may be what took out that resistor.

I can see it's some kind of senor against smooth glass with wiggley lines embedded in it...?

That would be a photocell. Look in the other end and you'll probably see a neon or incandescent lamp.

[eleventeen]

Anybody need some OD3 = 150 volt octal gas regulators
or 0C3 = 105 volt octal gas regulator, please PM me.

I have dozens of these and they have to go "elsewhere".
Free, DURING THE MONTH OF JULY, WHILE THEY LAST, if you pay domestic freight.

NOS WW2 surplus.

[dude]

That fried grid dropping resistor in above picture looks like a 10 watt cement power resistor.

I'll replace the leaking filters, I don't have a 5K 10watt, I have a 5K 5 watt or:

Looking at schematic's from Ampegs with 7591's, I notice that they have either a 470 ohm or 1K (10 watt).

I have a 560 ohm 10 watt cement resistor, think that would be OK?

al

[Willabe]

I have a 560 ohm 10 watt cement resistor, think that would be OK?

Yes, that should be fine. Some guy's like a larger value for the screen R for more compression and/or for more stable control of the screen grid's dissipation. Get it up and runnin and adjust later if you want.


     Brad          

 

[HotBluePlates]

So, Gibson probably just put this circuit in a case that says AS100.....?

Could be. Gibson did a lot of stuff without documenting it (or they kept it in house). HotBluePlates may have some info.

I can see it's some kind of senor against smooth glass with wiggley lines embedded in it...?

That would be a photocell. Look in the other end and you'll probably see a neon or incandescent lamp.

Well, I worked at Gibson about 30 years too late to have seen this strangeness. Either way, I dealt almost exclusively with guitars over there.

Sluckey, you and I can split the prize on this one...

Compare your amp to the Epiphone EA-65 schematic. I went through a ton of the Gibson Master Service Manual to find amps with 7591's. Well, the pair of reversed diodes in your picture, as well as the internal pot guided me to the EA-65.

It looks to me like the Epiphone amps might have been disappearing, or somehow an EA-65 chassis when into a GA-100 cabinet. Either way, the strange circuit is explained by the title page of this amp in the manual: "... with Photon Power Control (patent applied for)."

This little doo-hicky is part of a feedback circuit from the OT secondary to a point between the volume control and the second preamp stage. It samples the output at the speaker, which is rectified by the reversed diodes and filtered by the 200uF caps and 68k resistor (to the left of the pot in your pictures). That results in a somewhat varying d.c. that lights the bulb, and can shunt your signal away between the first and second stage.

The pot sets the threshold for this circuit. It is marked "factory adjust" but I'm not sure how useful this is as a compressor or limiter if you can't tinker it yourself.

Anyway, looks like Sluckey was right that this mysterious red object is an LDR and bulb.

[sluckey]

Elementary, my dear Watson. HBP, you're the real sleuth!

And Dude, your burnt resistor is 1K.

[DummyLoad]

Elementary, my dear Watson. HBP, you're the real sleuth!

And Dude, your burnt resistor is 1K.

looks kind of williamson-ish no? the cathodyne then the gain stages for each phase.

kick that thing in the pants, drop a couple of bypass caps under both sections of V2, make one switchable for some added overtones/harmonics.

[dude]

Thank you, you guys are awesome.

It is indeed an Epiphone EA65 Rivoli

Once I get it running, I guess I can turn that pot on the "factory adjust" Photon Power Control, to suit my ears or should I just heat shrink it and leave it?

HBP, thank you for taking the time to find that schematic, looks to be exactly what I have. When I first took the bottom off the head I never even looked at the guts or tube layout just went for the AS100 schematic but when I saw the original 7591, WT?

I wish the amp was mine but it's a friend's, friend. He paid $10 for it at a yard sale (I never get lucky but I get to play it a little bit and it's fun fixing amps) it worked till the leaking caps blew the screen rail resistor...

thanks all again,

al

[HotBluePlates]

You can probably wrap the LDR and bulb in tape to seal in the light. As far as the rheostat adjust, set it where you like and forget it. Or, drill a hole in the chassis and replace the stock pot with a typical chassis-mount pot and knob for an external (but obscured) adjustment.

It can only reduce the output of the amp. It doesn't exactly reduce the output power capability of the output stage, so in my mind, that makes it more like a radio's AVC, except it doesn't tinker a tube's gain but just throws away signal to ground to reduce the level at the speaker.

[PRR]

I hope this unit has a proper 3-pin power plug, not that "Gnd clip".

> looks kind of williamson-ish no?

No.

Yes, it IS a stunningly strong driver for a final that only needs 20V drive.

But the core concept of the Williamson is high NFB for low THD. This has no audio NFB.

> Photon Power Control

This is for your Photon Torpedoes.

> I guess I can turn that pot on the "factory adjust" Photon Power Control

It is perhaps the wackiest rectifier I will see today. I do fear that if pot is turned to the zero resistance point, the internal lamp could burn-up. OTOH it may already be burnt. That may be fine.

It is very strange this gimmick can't be turned-off.

> in my mind, that makes it more like a radio's AVC

It is a bizzare "limiter". A normal limiter turns-down all loud parts. (Radio AVC makes all medium to loud stations the same level to the volume control). This one only turns-down the beginning of a loud part. On guitar, it softens the initial pluck, about 25mS worth, then opens-up to a degree set by the pot. If they wanted "normal" limiting, the two 200uFd and two diodes would be pointless (except in that they may appear to evade the patent on most useful LDR limiters?). It also works at a set OUTput level... if you play small and soft, it does nothing.

Personally, after wasting a month restoring and studying this silly Photon Power, I'd probably disconnect the un-grounded leg of the LDR (smooth glass with wiggley lines) to defeat it. If it does anything, it isn't something ANYbody thought was worth copying.

> 7591 ... fixed bias too

7591 is an odd duck. A high-gain 6L6. When the amp becomes yours, and you tire of messing with 7591 (old ones are rare and new "7591" are only close-kin), convert to 6L6. Do this: add 1 ohm 1 Watt cathode resistors under each output tube. The 390K off one rectifier plate, change to 220K 2W. Change both 20u bias-filter caps to 22u 50V. With output tubes removed, read voltage at "-20V" point. This needs to be 40V to 45V to keep 6L6 from red-plating. For smoke-test, adjust the nearby "51K" to get -45V or -50V. Put voltmeter on a 1 ohm resistor. Insert 6L6 in that socket. Hit switch. It should warm-up to no more than 50 milliVolts (50mA in 1 ohm). 20mV is cool but safe for test. Check other tube in other socket. If both tubes will idle near 20mA-50mA, then play. If it plays well, fine. If idle was closer to 20Ma it might play well loud but rough on very faint passages, diddle the "51K" to get 40mA each tube. Beat it and caress it for an hour, re-check the bias.