Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: blueguitone on December 15, 2010, 05:30:59 am
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wanted to say hi i just joined recently.
and also i recently bought a Epiphone Pacemaker EA-50T and have been looking for any mods online and have not found any for this amp. does anyone know of any places to find anything for this amp.
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Not much you can do with such a simple amp. Do you like it? Got good tubes in it?
I attached the schematic.
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If the filter caps are original, you'll want to replace those.
You can get the parts you need from Hoffman.
You could consider lowering the cathode cap on the first gain stage to 5uf and adding a cathode cap of either 5uf or 2.2uf to the second gain stage of the 6EU7.
The large 250uf cap coupling the phase invertor seems odd to me? I'd probably do a coupling cap that is more like on a Silvertone 1482 or blackface Princeton.
You could increase the 20uf cathode cap on the power tubes to 47uf if you want a little more gain and bass.
Other than that I'd leave it alone.
With respect, Tubenit
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thanks fellas,
You could increase the 20uf cathode cap on the power tubes to 47uf if you want a little more gain and bass.
i should have mentioned these two things were pretty much on my mind in the first place, thanks a lot.
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I'd suggest only making changes if there is a particular problem with the amp that you want to address.
That said, compare the schematic LC posted to the 5E3 Deluxe. You will see the same number of tubes in a single channel of the Deluxe, and recall that a 6AQ5 is basically a 6V6 in a 9-pin bottle, with a 6EU7 being basically a 12AX7 with a different pin-out.
But you said EA-50T, and your amp has more controls than indicated on the EA-50 schematic. Does you amp's innards resemble this EA-50T schematic (http://www.el34world.com/charts/Schematics/files/gibson/ea_50t-pacemaker-trem.pdf) more closely?
Is your amp overly bright? Do you play guitars with single-coils or humbuckers? Does your amp have 1 6EU7 and 1 6C4 (or 12AU7), or is it a pair of 6EU7's?
By knowing which amp you have, I can tell you how to convert it to essentially a 5E3 Deluxe (with or without trem), if that is the direction you want to move in.
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But you said EA-50T, and your amp has more controls than indicated on the EA-50 schematic. Does you amp's innards resemble this EA-50T schematic (http://www.el34world.com/charts/Schematics/files/gibson/ea_50t-pacemaker-trem.pdf) more closely?
yeah it's the EA-50T
Is your amp overly bright? Do you play guitars with single-coils or humbuckers? Does your amp have 1 6EU7 and 1 6C4 (or 12AU7), or is it a pair of 6EU7's?
it has a pair of 6EU7's, i have strats only, yeah it's bright sounding, also i would like it to break up earlier.
By knowing which amp you have, I can tell you how to convert it to essentially a 5E3 Deluxe (with or without trem), if that is the direction you want to move in.
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i was just checking out some of the different videos on youtube the 5e3 is a really cool sounding amp and getting in that direction would be something i would be interested in.
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Remember that Gibson built mostly humbucker guitars, and you'll understand why their amps are often voiced to be very bright.
You will want to start by nullifying the T-filter tone network. Refer back to that EA-50T schematic. The T-filter is made up of C3, C4, R11 and R12. You could pull these parts out, but if you simply connect a jumper (could be a wire with alligator clips on each end) from the junction of C2 (0.005uF coupling cap) and R11 over to the junction of R12 and R27 (the non-grounded outside lug of the volume control), you will short out the T-filter and bypass it.
This is a good step, but you'll also want to beef up coupling cap C2. It is presently a 0.005uF cap, and strips out a lot of bass. Make that thing between 0.01-0.022uF instead. That will change the bass rolloff to just about at the bottom of the guitar's range, or somewhat lower, and will allow your guitar's fullness through.
There are other issues in the non-trem EA-50 circuit that hold it back gain-wise. That's not so much the case with your amp. You could raise C5 (the 2nd stage bypass) to 1-5uF to bring back some fullness to your amp's sound. You can go as high as 25uF or more, but you don't need to do that to get a full sound and full gain.
Don't worry about the seemingly small value of C6 (the coupling cap feeding the phase inverter V2A). Due to bootstrapping on the input of the phase inverter, the apparent input impedance is very large, and doesn't require a large coupling cap for full bass.
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Wow, a 6C4! Used to be a very widely used single-triode tube in RF stuff, its RF specs are reasonably impressive for the time; the garage of doom yielded far too many of them. In some ways it's surprising to see a 6C4 being used in the late-50's/60's. Maybe the (very unfortunately) ailing Epiphone got a good deal on them. That amp resembles a Danelectro in some ways.
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Wow, a 6C4! Used to be a very widely used single-triode tube in RF stuff, its RF specs are reasonably impressive for the time; the garage of doom yielded far too many of them. In some ways it's surprising to see a 6C4 being used in the late-50's/60's. Maybe the (very unfortunately) ailing Epiphone got a good deal on them. That amp resembles a Danelectro in some ways.
bogen used the 6C4 triode in some of their PA amps as a split load PI. CHB-50 and CHB-100 are classic examples. it is 1/2 of 12AU7 in a 7 pin miniature.
sorry for the thread-jack... converting to 5E3 would be taking a most delicious sounding direction. HBP is just the man to have as your guide. :smiley:
--ISO
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hey thx for all the great info HBP, if i get a day off anytime soon i plan on working on this amp. :cry:
don't worry about any hijack i'm open to all info and ideas. someone gave me this amp, so for no money into it i'm pretty happy with what it has to offer, and what can be done with it.
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>Wow, a 6C4! Used to be a very widely used single-triode tube in RF stuff...
>bogen used the 6C4 triode in some of their PA amps as a split load PI.
Funny how this stuff runs in packs sometimes. I just did some work with a Hammond AO-41 vibrato unit that used the 6C4 as a cathode follower/driver.
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Wow, a 6C4!
Scroll down to the schematic I linked of the EA-50T.
It is funny how the EA-50 uses a 6C4 (call it half-a-12AU7), as well as a split plate load to reduce stage gain, but the EA-50T uses a pair of 6EU7's and no gain reduction at the plate load resistor.
Gibson musta owned Epiphone by the time these amps were released... It bears all of Gibson's hallmarks with noticeably different circuit with same/similar model numbers.
Or maybe the trem version had a higher price tag, so they upped the gain slightly along with adding an effect to get the extra money.
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One other mod that may or may not be something to think about, as the 6EU7 is essentially a 12AX7 in a different pin-out and only a 6v heater option AND is a more expensive tube, is to rewire for use of 12AX7's instead. You don't have to do this, but cost wise may be something to think about. OTOH... no harm in keeping it as-is either, I got a Davis PA amp I converted (PtP, I gutted and rewired) that uses them and I left them as-is.
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Yes it's pretty easy to convert this amp to something more like a 5E3.
Personnaly, I would just jumper the T Filter, and add a big bypass cap on the cathode of the 2nd stage (as HBP said).
I wouldn't touch the rest. Small coupling caps are nice IMO, they really help remove the flubiness if the amp is pushed (and I don't play humbuckers).
However, the amp might be a bit cheaply made, and it might not like being played loud and distorted for a long while (especially if they used 1/2W or 1W power rail resistors).
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The main reason for the suggestion to raise C2 is that the -3dB point is around 400Hz. That could be pretty bright with a single-coil guitar if you're not pumping in a lot of distortion (maybe through a pedal) and need to reduce the low end to keep the amp from being muddy.
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Long time reader - first time poster.
I've got a Pacemaker EA-50T that has a hum. With the volume down, no hum. Very silent. Even with no instrument plugged in, as the volume is brought up, the hum comes up. I had removed the tone filter circuit years ago and I replaced it with a .01 mfd. It had always had that hum and that's why I had put the amp away. Replaced C1 - no help. If I disconnect the .01 to the volume control, the hum goes away. It has to be coming in from the first 6EU7. Replaced the 6EU7 just for the heck of it - no help. All other large caps have been replaced and checked. I feel like the hum is coming from the filament circuit. Other than that, the amp sound great.
Any suggestions would be greatly appreciated.
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I feel like the hum is coming from the filament circuit.
measuring it is best, 60hz most likely filaments
fwiw;
I've found the sockets on G amps to be part of the problem, rat-nest wire, part, single-wire fil, part
do 1 change, then another, then it's gone.
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I feel like the hum is coming from the filament circuit.
measuring it is best, 60hz most likely filaments
fwiw;
I've found the sockets on G amps to be part of the problem, rat-nest wire, part, single-wire fil, part
do 1 change, then another, then it's gone.
I don't understand this part of your reply... and thanks for your reply.
"part do 1 change, then another, then it's gone."
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sorry, try this;
Identify the hum frequency
check, tension, clean, the tube socket
thump on the tube itself to "check" for microphonic's
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sorry, try this;
Identify the hum frequency
check, tension, clean, the tube socket
thump on the tube itself to "check" for microphonic's
Interesting. I recorded the hum to a WAVE file using a handheld recorder. Analyzing the file, the frequency of 120 Hz is the strongest on the spectrum. The socket is clean, the tube is seated well and I tried another 6EU7. No microphonics.
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here's what I meant by try one, try another, ......
called the "shotgun" method :icon_biggrin:
all the red circles should be replaced one at a time, or maybe 2-3, depending on your confidence.
use new, metal film resistors, and a new Ecap (if you haven't replaced it already)
maybe start a new thread for your specific problems
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I like these amps. I have a 50T and a Gibson of the same circuit with no tremolo (one of the GA-5 Skylark versions)
Anyways - I had a similar hum in my 50T - turned out that the heater circuit ground connection had worked itself free. These (or at least mine) do not have twisted pair, they are series with a connection to ground at V1. Reconnected that - hum gone.
And, once you fix the hum, I suggest you check to see if the amp matches a known schematic. Mine both had the infamous Gibson T filter even though the schematic did not include it. Getting rid of it really helps. Gibson played fast and loose with schematics/circuits. Once dialed in, these amps have a really cool overdrive with those 6AQ5s.
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schema for a EA-50T - has the T filter.
--pete
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Use some clip leads to connect a 20µF cap across C16A. Any better?
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I appreciate the replies. I have done all the suggestions. Jumped all replaced electrolytics just to make sure new ones are good. Replaced all components in the first 6EU7 circuit. This amp has always had this hum as I stated in my original post. I think it needs an exorcism.
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schema for a EA-50T - has the T filter.
Yup, I likely have the Gibson and Epi versions of the schematic confused.
But, what I am sure about is that the hum I had in one of those amps was as you described. I tracked it to the heater circuit, which I had at first wrongly thought was ok. So going with your intuition that its the heater wiring, you may want to make absolutely sure that is OK. One of my two amps had a skimpy little solder pad ground at the V1 socket for the heater circuit. That connection was bad - and very hard to see. Fixed that and hum was gone. Of course the issue with your amp may be completely different, but just passing that info along.
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schema for a EA-50T - has the T filter.
Yup, I likely have the Gibson and Epi versions of the schematic confused.
But, what I am sure about is that the hum I had in one of those amps was as you described. I tracked it to the heater circuit, which I had at first wrongly thought was ok. So going with your intuition that its the heater wiring, you may want to make absolutely sure that is OK. One of my two amps had a skimpy little solder pad ground at the V1 socket for the heater circuit. That connection was bad - and very hard to see. Fixed that and hum was gone. Of course the issue with your amp may be completely different, but just passing that info along.
Yep. TNX. I saw that pin 9 heater ground and checked it. Solid as a rock. The single ended filament wiring sure is weird. I'm so used to working on Fenders. Maybe this hum is normal, yet I see others complain about it. Sure is a sweet sounding amp. Glad I brought it out of the vault.
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Well, I'd say that if its loud enough to be annoying, its not normal hum. I find it very frustrating to replace caps and resistors - getting my hopes up - and then having them dashed :BangHead: So when I get to the point you are at, if the noise is coming through the speaker, I set up a listening amp and probe along the signal chain. At times I have been surprised at where a noise was coming from.
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Yep. TNX. I saw that pin 9 heater ground and checked it. Solid as a rock.
Pin 9 is not a heater pin for any tube in that amp.
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Yup - heaters are 1&2 on the 6EU7 - my recollection is that its pin one V1 that was jumped to ground.
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Yep. TNX. I saw that pin 9 heater ground and checked it. Solid as a rock.
Pin 9 is not a heater pin for any tube in that amp.
Yeah yeah... I meant pin1