6CG7 substitution tube ?

[kagliostro]

A long time ago I was asking about the substitution of an ELL80 tube in a '60 Italian Davoli tube amp

(there the ELL80 tube was used as PI and connected as triode)

the perfect match is a pair of EL95 tubes, but a friend used a different way, he used a 6CG7 tube

now I have necessity to find an equivalent tube for the 6CG7 tube

have you a suggestion ??

Many Thanks


K

[HotBluePlates]

The 6CG7/6FQ7 is like a double-power 12AU7. Except for the doubled plate dissipation and higher cathode current ratings, the tubes are similar for voltage gain and internal plate resistance.


If you don't need the power output (the tube is being used as a phase inverter), maybe the 12AU7 could work?


Otherwise, the EL95 seems cheaper than the 6CG7 in Europe... Despite the high asking price from some tube vendors, the 6CG7 doesn't seem to have many places it's being used today and often turns up quite cheap at hamfests and the like.

[kagliostro]

Thanks HotBluePlates

what do you think about an ecc99 ??

http://www.jj-electronic.sk/pdf/ECC99.pdf

attached the schematic of the power section of the amp and the mod who was done using 6CG7 tube

---

The price for the 6CG7 tube at TAD is really affordable, Thanks I ignored that TAD has those tubes




Franco

[HotBluePlates]

Any idea what the bias voltage is for the output tubes? That should dictate the requirements of the phase inverter tube. I don't see anything that requires power. It looks to me like internal plate resistance and amplification factor are the big concerns. Match those and everything would be fine.

[PRR]

> I don't see anything that requires power.

The plan "DAVOLI U.P. 80.gif" shows *33K* grid resistors on the EL504s.

Hence 10K plate resistors on the driver tube.

It is not clear why they used 33K grid leaks. The data sheet says 500K is legal.

ELL80 is a twin pentode but here strapped as triodes. That sheet does not give triode data but page 4 shows about 15V to cut-off with 250V G2, so Mu around 17.

Squinting at clues, I'd guess it runs near 450V on EL504 plates, 225V on G2, 350V supply to driver. Maybe 200V on ELL80 plates, so 150V across those 10K resistors, 15mA each side, 3 Watts per plate.

Without a clue to WHY they used 33K, if I "copied" this, I'd go 100K (and reduce 2ufd to 0.6uFd), change 10K to 33K, up the 390 ohm to get near 200V on plates. Now we are at 4.5mA and 0.9W per side, a 12AU7 is plenty good.

R14 needs trimming for any change in V2 (if you demand "best THD"). The range seems to be ample for a change from ELL80 to ECC82 or 6CG7 or 12AU7...... argh!

Note the very low impedance of the R14 etc leg. It does not have to be low, it feeds a small grid. This also should be scaled-up at least 3X. Say 100K pot, 100K fixed, 10K-15K fixed.

*MAYBE* the output transformer is VERY extended frequency response. Past 50KHz. Then the low-low impedances make sense. Is this a repair (don't change what does not have to be changed) or a "reproduction" with available OT (which is unlikely to have fantastic high frequency)?

[kagliostro]

I Apologize
Yesterday I attached the UP80 schematic I have on the database of my PC, but I would like to attach one other version (official) of the schematic, but as I have that schematic saved on the web and I had difficulties on connecting, I posted the schematic I had under hand

Here attached the official schematic

Now I go to read the new post you sent

Sorry for the drawback HotBluePlates and PRR 

Franco

[kagliostro]

Did I miss something on the hand sketch, the power tube cathodes do not appear grounded or tied to anything

Yes, seems they begin to draw the ground symbol but forgot to complete it
K

[kagliostro]

Despite the high asking price from some tube vendors, the 6CG7 doesn't seem to have many places it's being used today and often turns up quite cheap at hamfests and the like.

After 9 years of Flea Markets, yesterday at a Radio Amateur small Ham Fest I've find 4 x NOS 6CG7 really cheap, 10.00€ for all (2.5€ each)

patience is never too much

 

Franco