Epiphone EA-12RVT pics and schematic wanted

[bepone]

started today with full tube set, managed to get acceptable tones on reverb channel (more thick), other channel is little bit thin.. treble pot there is working extreme so it is difficult to get normal setting..
little bit hum is permanent, not dependent from the signal..need to check gnd's

ga45.jpg

[bepone]

managed to get low noise on the amp.. tone is still not so, i will remove T filters for the test..
tremolo working up to the roach, after that no more
will be checked too..

[bepone]

replaced roach.. and ECC82 was dead on the heater line (half heater working, half not on the NOS , new tube)... i never had tube with only half heater operational , but always need to be first time
probably this tube was rejected in old days, but sold like a NOS nowadays of course...

tremolo-vibrato working, tommorow i'll see for the reverb

[bepone]

reverb also operational... vibrato after mixing with the rest is very limited.. or roach is not so ok, or some other problem..today i have dismantled one bug, but fell apart alone.. amp now taking a lot of time for limited or no progress at all.

little bit thin sound is still here, today i have removed one T filter, maybe i will proceed with the others.. this is PP amp without feedback, i'm thinking to add shade NFB, is it ideal amp for that , from output anodes to previous stage anodes, 470k 5W for the start , to make heights more friendly.

[bepone]

trem roach,LDR part is alive, tomorrow will be rebuilt with some LED or neon bulb

IMG_20230503_230029__01__01.jpg

[Phil_S]

You should try it, but I don't think LED or neon will work. Look for a low voltage, low current incandescent. You might look in an automobile parts store for dashboard bulbs. I'd very much like to know what happens and hope for success.

[bepone]

i hope,
this morning tested oroginal lamp ok, and ldr ok, probably contact was faulty....
so i decided to resolder all and put back in the tube, resaled with epoxy..

IMG_20230504_091652__01.jpg

[bepone]

LDR roach put back in the circuit, to monitor what is going on inside, neon bulb externally installed also ..adjusted bias to tremolo tube, observed normal tremolo operation, after mixing in circuit also ok.
After a lot of troubleshooting, circuit is quite critical with voltage, if undervoltage (low B) it wont work.. must be good working voltage..neon bulb is firing at 60-110V.. here we have 130V.. AC component is only 40V.. so i needed to increase B+ voltage, increase of gain of the stage (poor ECC82, 12AU7) will be better some other tube.. and setup bias to get desired depth in the tremolo..

Fender doesnt have this problems, trem there is working at 250V+

1234.jpg

[Phil_S]

... here we have 130V...

Yes, this is why USA type incandescent is needed. Auto parts suggestion was wrong as those are 12V. Finding the right bulb is not so easy but possible. Maybe in Croatia, where I assume you have 230V mains, finding a 120V bulb is nearly impossible. Voltage modification is a good idea if you can actually do it.

I don't know if you can reach this web page. I think this is a good example of a bulb that will work. It is 120V, 25mA, 7.11mm diameter, 27.94mm length. I think it will fit inside the black tube.
https://lightbulbdepot.com/products/det ... roducts%2F

Were you able to identify the LDR spec? That one has always been a mystery to me.

It sounds like you have made good progress.

[bepone]

I think bulb of 50V will be good! They are on DC node, and powering up only with AC signal..
Original bulb is starting to work also on 20-30VDC i have tested with DC supply,

LDR is working, dropping from Megs to 2k when is full light on it..

[bepone]

LDR is strange type , molded with the glass
i think it can be easilly made DIY nowadys

[bepone]

Unhappy still with the work of original tremolo (too much current through the ECC82 -10mA, and depth in tremolo effect, sluggish start and stop which makes tremolo blurry) finally dismissed original roach,
and installed Vactrol VTL51C optocoupler, over 390k (in series with LED), resistor chosen for the effect.

Now it is working ok and i will leave it.

For reverb- managed to repair reverb, but is extreme - 1/10 is allready too much, so gain in reverb tubes has been reduced to 25%, and installed divider 5/1 after reverb amplification.. now it is correct and fine,

Removed all the T filters, amp starts to sound very good, noise is low

[Phil_S]

...installed Vactrol VTL51C optocoupler, over 390k (in series with LED), resistor chosen for the effect. Now it is working ok and i will leave it.

Yes, I've been thinking this might be a good choice. They used to make these with incandescent lamps, but those haven't been available for at least 10 years, maybe more. Glad to hear this works! And thanks for the info on the LDR.

Removed all the T filters, amp starts to sound very good, noise is low

Yes, good move. I should do that to my GA20-RVT. It just sits in a closet.

It sounds like you've made great progress on this amp.

[bepone]

Yes, I've been thinking this might be a good choice. They used to make these with incandescent lamps, but those haven't been available for at least 10 years, maybe more. Glad to hear this works! And thanks for the info on the LDR.

Yes, good move. I should do that to my GA20-RVT. It just sits in a closet.

It sounds like you've made great progress on this amp.

Yes after few weekes of painful job but resulting with not emusing sound however the end is happy end.. noise is very low, considering all the wires from the gain pots vipers inside at least half meter each, unshielded (!!) reverb is working so nice that has this space feeling in it, vibrato is normal and with good depth, cleans are chimy but not shrill..better sound than nowadays digital standard , by far!

Removing the T filters resulting of increasing the overall gain a lot, maybe i will kill little bit of overall gain in the amp.. also opens a lot the bass side and treble too.

I have another chassis fully assembled /without transformers/ like a donor amp.. now i'm tempted to complete second one too

What is special also , this is no feedback amp, all is coming out there is no supression to any harmonics and you can hear that..overall balance - sound is on presence side little bit but is not a problem