Vintage Epiphone combo

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andresound
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Re: Vintage Epiphone combo

Post by andresound »

Played the amp for an hour last night. Really good sounding amp (it has MOJO) :lol: Awsome sounding reverb!! I will go through the entire circuit this weekend and check every component and voltages.

Few things I noticed.
1. Although the amp had a bit of paperwork with it, which states it is a EA-10RV, the circuit looks to be a GA-77RV.
2. At turn on, the amp has a slight bit of mains hum, which is constant.
3. The filter caps in the amp I will change, but.... The values of the big yellow ones in the pic and the other ones are very different to the GA-77RV schem. The first filter cap before the recto is shown as a 20uf, and the following ones are 10uf. All the caps installed are higher.
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Phil_S
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Re: Vintage Epiphone combo

Post by Phil_S »

andresound wrote:Played the amp for an hour last night. Really good sounding amp (it has MOJO) :lol: Awsome sounding reverb!! I will go through the entire circuit this weekend and check every component and voltages.
That's awesome news!
Few things I noticed.
1. Although the amp had a bit of paperwork with it, which states it is a EA-10RV, the circuit looks to be a GA-77RV.
Typically there is an equivalent Epiphone model to each Gibson. Minor variations are known to be rampant from this manufacturer. I have a Gibson GA20-RVT of which there are two "known" versions, of which I have the 3rd. I you have a match on the tube compliment, general circuit (i.e., both have reverb,similar tones stack, etc.), transformers, speaker, and cab, I'd consider it a match. If you can't determine some of this, go with tubes and general circuit for a match. Sometimes transformer part numbers appear on the schematics, sometimes not.
2. At turn on, the amp has a slight bit of mains hum, which is constant.
Not surprising in an amp this age. It probably needs a grounded power cord and a cap job. It is surprising what these two routine maintenance items will do. I'd suggest changing ALL of the electrolytic caps on account of their age.
3. The filter caps in the amp I will change, but.... The values of the big yellow ones in the pic and the other ones are very different to the GA-77RV schem. The first filter cap before the recto is shown as a 20uf, and the following ones are 10uf. All the caps installed are higher.
The originals would probably be wrapped in brown paper with a metal band riveted to the chassis. See the picture. If they are original, just stick with the values. If not, there is nothing wrong with 20-10-10 (or whatever). All 20's seem like a good choice to me. 20uf, 22uf, and 25uf are all functionally equivalent in this instance, so buy whatever is most appropriate for your budget and with respect to size/style for fitting them to the amp. Pay attention to voltage ratings as you don't want to go below what you see on the B+ rail and you want some headroom above the B+ rail voltage. Any new cap you buy will be of considerably higher quality compared to what the amp was built with.

Seems like you have a great find!
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Firestorm
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Re: Vintage Epiphone combo

Post by Firestorm »

A 20uF main filter might be a bit light for a pair of 6L6s (unless you like that underfiltered ghost note thing). The 77RV gets away with it because the choke is first in line. Does yours have a choke? Where is it located electronically?
andresound
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Re: Vintage Epiphone combo

Post by andresound »

Thanks for the input guys!! I will verify all of their above his weekend and then post. I do have 20uf caps on hand, so will do as suggested. Maybe I can beef up the first cap (20uf), which appears between the choke the GZ34. Maybe double it?? The GZ34 which I just put in (NOS Mullard), should be OK with the inrush? As it stands, a little more headroom would be good (I think these old ears are tired :lol:)

This amp is supposedly a 25 w design. My TW rocket (30w) and the TW express when fitted with 6v6 (22w), seem much louder and have more headroom. Although the Jensen 15" is only 96db/1w/1m.
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martin manning
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Re: Vintage Epiphone combo

Post by martin manning »

I think doubling the first filter and perhaps the second (screen) would be fine. The other three could stay at 10uF. I'm interested to hear if the circuit does indeed match the GA-77RV, and how you made that association.
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H

Post by Stevem »

Volume and punch are two different things and many times the Bass roll off some of these amps had built into , or the roll off due to value / age related components is the reason for your lack of punch comment.

Those vintage Top of the line Jensens , like the P15N and the later C15N had the biggest power rating out of all the Jensen line in there concert series and even then it was only 25 watts and the 45 plus year old paper voice coil formers that they used back then are iffy even at a 20 watt rating before they turn brown and crumble.
Remember these amps where not made to be played to the point of clipping in the output stage!

In short if you like the tone of the speaker don't push it too hard , or get a new reissue from Jensen that will live with more power should you find it!

Also, do not forget that a steady mains hum can be due to the matching on the output tubes.
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andresound
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Re: Vintage Epiphone combo

Post by andresound »

martin manning wrote:I think doubling the first filter and perhaps the second (screen) would be fine. The other three could stay at 10uF. I'm interested to hear if the circuit does indeed match the GA-77RV, and how you made that association.
Martin, I have a feeling you already know the answer to the above :lol: I believe I am "jumping the gun" in my assumptions. I just assumed because the tube complement and use of a 15" driver was identical, that it must be that circuit Being stupid in things technical (but I WILL eventually improve), I see that a GA-77 circuit is fixed bias and the EA-10 is cathode bias which this amp definitely is. Am I correct in this??
Martin, maybe I will trace out this circuit and try draw it. I believe this would be a good educational exercise for me. This might take a few years (LOL), so bear with me.

Stevem, thank you once again for your input. I am sure I saw somewhere that the 15" Jensen in this amp is rated at 50w??? Anyway, still a long way to go. I will replace the lytics and check all component values, then try out a pair of matched 5881's I have in my ODS and see if the "hum" improves

I must admit, I really do feel like an idiot amongst you guys. Actually, come to think of it..... THAT IS A FACT!! :oops:
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pdf64
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Re: Vintage Epiphone combo

Post by pdf64 »

andresound wrote:At turn on, the amp has a slight bit of mains hum, which is constant
Does the hum start immediately the PT is energised, ie even before the tubes have warmed up?
pdf64
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Re: Vintage Epiphone combo

Post by pdf64 »

martin manning wrote:... To confirm the output tube type, measure the OT primary impedance. For 6L6 it will be around 4k, and for 6V6 it will be around 8k.
See p4 of
http://www.mif.pg.gda.pl/homepages/fran ... /6L6GB.pdf
and
http://www.mif.pg.gda.pl/homepages/fran ... /6L6GC.pdf

The EA10 power tube voltages look fairly close to the cathode bias typical operation values in the rh column.
So it could be a 9k primary yet still be intended for 6L6GB/C!
Last edited by pdf64 on Fri Aug 19, 2016 7:39 pm, edited 1 time in total.
Stevem
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Speakers

Post by Stevem »

New Jensens made in Italy are rated at 50 watts , not the original ones!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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martin manning
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Re: Vintage Epiphone combo

Post by martin manning »

pdf64 wrote:
martin manning wrote:... To confirm the output tube type, measure the OT primary impedance. For 6L6 it will be around 4k, and for 6V6 it will be around 8k.
See p4 of
http://www.mif.pg.gda.pl/homepages/fran ... /6L6GB.pdf
and
http://www.mif.pg.gda.pl/homepages/fran ... /6L6GC.pdf

The EA10 power tube voltages look fairly close to the cathode bias typical operation values in the rh column.
So it could be a 9k primary yet still be intended for 6L6GB/C!
EA-10 schematic voltages (intended for 6L6GB) are a bit lower at 323 and 264. Using the plate curves and the EA-10 voltages I'd say 6k6 would be just about right, but you never know. Maybe Andre will measure it and report... ;^)
andresound
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Re: Vintage Epiphone combo

Post by andresound »

martin manning wrote:It'll be an interesting ptoject for sure. To confirm the output tube type, measure the OT primary impedance. For 6L6 it will be around 4k, and for 6V6 it will be around 8k.
Is it possible to measure the OT without disconnecting the OT and applying AC to figure out the turn ratio and then calculate the impedance? I have never done this exercise before!
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martin manning
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Re: Vintage Epiphone combo

Post by martin manning »

andresound wrote:
martin manning wrote:It'll be an interesting ptoject for sure. To confirm the output tube type, measure the OT primary impedance. For 6L6 it will be around 4k, and for 6V6 it will be around 8k.
Is it possible to measure the OT without disconnecting the OT and applying AC to figure out the turn ratio and then calculate the impedance? I have never done this exercise before!
Yes. With power off and the speaker disconnected you can put some AC on the primary (plate to plate), and measure voltage on the primary and secondary. The voltage ratio is the turns ratio, and turns ratio squared is the impedance ratio. Speaker impedance times impedance ratio is the primary impedance. It's best to put a load resistor (matching the speaker impedance) across the secondary when making this measurement.
andresound
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Re: Vintage Epiphone combo

Post by andresound »

Removed amp from cab today. The OT is marked "GA-70". The schem suggests this is for a 5881 fixed bias output stage.
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andresound
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Re: Vintage Epiphone combo

Post by andresound »

OT looks small to me :? . Herewith pic of chassis and PT + reverb driver
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