Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: LooseChange on June 25, 2010, 07:59:08 pm
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I have a Valco Chicago 51 (1953 dated) on the bench.
I finally found the Schematic cross referenced to a Gretsch G6155. Interesting enough the schematic was drawn incorrectly but I fixed it and it's posted below.
On the front of the cab it says "Supro".
Pictures...
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Okay, here is the schematic and question...
Should there be voltage on the grids of the two power tubes (pin 5)? I think not but I'm not very good with Phase Inverters... Still learning these old ones.
Thanks!!
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Okay, here is the schematic and question...
Should there be voltage on the grids of the two power tubes (pin 5)? I think not but I'm not very good with Phase Inverters... Still learning these old ones.
Thanks!!
IRRC that's called a "paraphase" PI. TUT 1 page 6-12 calls this the "old type ca. 1930" vs. a "new type ca. 1942". The difference is that the voltage divider off the plate of the first triode (pins 2 & 3) goes to "actual" DC ground instead of using a "virtual ground" (AC ground but not DC ground) which allows local NFB.
Ignoring all that, the .05 coupling caps after the PI plates should be keeping all DC off of the power tube grids. Unless I'm reading the schematic all wrong...
Cheers,
Chip
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Right. There should be a.c. signal, but no d.c. voltage. If you have d.c. present, the coupling caps are leaking.
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Should there be voltage on the grids of the two power tubes (pin 5)? I think not but I'm not very good with Phase Inverters... Still learning these old ones.
Thanks!!
Schem shows a cathode biased power stage, so there should be no D.C. on pin 5 of power bottles. Lift the intersection between P.I. coupling caps - 0.05uF + 270K - and check for D.C. leakage.
Hope this helps
Best Regards
Rzenc
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a buddy brought me a Valco Chicago 51 to replace the speaker. Its different than the unit shown in the pic. It has two 12AX7 tubes and the power transformer is on one side, not in the middle as the pic here shows. I dont see any other ID numbering on this amp.
He brought a new 4 ohm speaker, he said it was the correct impedance. I just want to make sure its correct if anyone knows.
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Heh...well, you're going to have two issues...one is that the old field coil speaker is the choke. There's *possibly* some way to mount a new little choke upon the frame of the new speaker but that is not a perfect certainty...and in some cases, you just can't...there are no flat surfaces on the new speaker's spider. Nobody can be in love with the idea of running +350 volts up the the speaker and back, either, but I guess it's been done for 60 years in the example you have and there you go. And, the output tranny is also mounted on the old speaker. So you have some mechanical issues to deal with. One approach I have used and it is not especially pretty AND the magnet on the new speaker might interfere...is to remove the bolts from the chassis-mounted transformer and fabricate some metal plates upon which the two new trannies (choke plus tranny) can sit. Drill holes thru the plate or plates and mount the trannies to that plate. Or better, if you have a single piece of metal (steel or alum) in which you can cut a square hole, then the existing end-bell of the power trans can stick through that and you build a "platform" upon which the new trannies mount. Not that easy. My desire would be to NOT drill any new holes in the chassis, and that approach would do the deal. Better to use the single plate with the rectangular hole...but then you have some metalwork to do.
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The transformer mounted on this speaker has a three wire input from the chassis then two wires to the speaker. Does it absolutely have to be mounted to the speaker frame? I was considering mounting it to the side of the cabinet then extending the wire to the speaker.
The speaker in the amp now was a replacement at some point, its a Radio Shack 10" speaker @ 8 ohms and it was actually made in the U.S.A., been a while since Tandy has made anything in this country.
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Could do that too. It's the output tranny, no doubt. Wires for: two plates, B+, two spkr wires. But then there's the field coil, the "magnet" for the speaker which the new speaker will not have. It's certainly possible to omit the choke and throw a resistor in there, but my point is, you have two small transformers to mount somehow. What I don't like about the cabinet side-mounting scheme is that in the event the chassis has to be pulled for future service, it doesn't come out as one piece and you have these heavyish things hanging off in space....and then it's even worse if you have a choke and an OT.
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4 Ohm (3.2) is very typical for the period and market.
Does it work with the RadioShed speaker, even a little? Then someone *may* have done something about the field-coil already.
The most likely replacement for the field-coil is a 1K 20W resistor. While the FC has inductance, costs also forced it to have resistance, a lot. It is in a pi filter: cap, coil-R, cap, so the resistance alone (with two caps) adds a lot of filtering. However this 20W resistor has 350V and throws 10+ Watts of heat: hard to mount it so it will cool yet not shock the cat or the user.
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I dont think this unit was original with the field coil speaker set up. I have looked at several schematics and some show a three wire to the output transformer as this one has.
I have not powered it up. I do extensive visual inspections first and fount one pin of one of the output tubes has disconnected from the tube.
I will post some pics of it later.
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My pics are too large for the site to post.
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These two may fit. It has the serial number tag still nailed to it.
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Can anyone help identify this unit in order to locate an accurate schematic?
The two output tubes are removed which are located to the right of the rectifier tube.
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Thanks!
The two output tubes are removed which are located to the right of the rectifier tube.
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12AX7 and field-coils don't go together.
They *could*. Field-coils were going out when 12AX7 was coming in. Radio makers (and g-amp makers) often used-up whatever was in the warehouse.
Trace the path from rectifier cathode to OT. I can see some of it, but you can see it better.
I'd be looking at Jensen old-style speakers. Not only the right tone for the period, but some of thier frames may still have OT mounting areas.
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Can anyone help identify this unit in order to locate an accurate schematic?
The two output tubes are removed which are located to the right of the rectifier tube.
RC_cola,
the SN# T35xxx points to 1959 or 1960 (believe it or not). The valco cabinet design is correct to that era as well, as is the tube compliment (2x12AX7 2x6973 and 1x5Y3).
You might not find an exact schematic, however you'll get very close if you look for a similar amp with tremolo. I'd start with a 1624T and see how far off your circuit is from that. All of the 6973 based valcos of that era seem to be similar to the 1624T. later, maybe in 1962, the 6161 replaced the 1624T.
As for the speaker load requirement: valco liked OTs with 7.5K load on the primary for 2x6973 amps. They made a few different OTs with different secondaries depending on speaker load. 7.5K:8ohm is most likely. They made a 7.5K:2ohm !! with twin 6x10" speakers as late as 1958 or 1959, so I wouldn't rule out some weirdness. If you want to know for sure: pull all the tubes and speaker connections, put a few AC volts on the secondaries (3-4vac), measure voltage on the primaries and do the winding math.
The OT does not need to be mounted on the speaker frame, you can use wood screws and put on the cabinet. it would be a good idea to ground the core/bracket.
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For vintage valco amps, http://www.vintage47amps.com (http://www.vintage47amps.com) is worth a visit. I like their speakers. They are pretty much specifically designed for that 51 that LooseChange started the thread with.