EL84 push pull amp with a 12 inch spk on the bench today,
Hohner Contessa CA-100
- Thread starter CJ
- Start date
Help Support GroupDIY Audio Forum:
what happened to the chassis? right here>
75 EI core on the output,
12AX7a preamps, vibrato, 12AU7 phase inverter
12AX7a preamps, vibrato, 12AU7 phase inverter
guts, ceramic coupling caps in preamp stages, no cathode bypass caps except vib. osc.
Oh that handmade schematic noodle is hot. How did it sound prior? What was the repair request?
customer says loss of treble, does not have the same mojo hand as it did before,
tubes check ok,
speed control should have an arrow through it, (500K pot)
Sano was involved in these amps, there might be an alternate model number for the same amp,
tubes check ok,
speed control should have an arrow through it, (500K pot)
Sano was involved in these amps, there might be an alternate model number for the same amp,
yes. 40/40/20/20 525
yes this uses an 18 K : 8 Ohm output, maybe that is why people like the sound,
#37 wire for 63ma idle current is really pushing it,
print>
#37 wire for 63ma idle current is really pushing it,
print>
I have a "goofy" factory fitted OT in my much cheapness high gain JCA22H head (designed by Mike Soldano made in China) 2 x EL84 with a 3.6K:8Ω/4Ω OT, decided it would be my 1st OT strip-down/re-build. The OT turned out to be a bit more than just a classic 3 section Pri|Sec|Pri job tho.
Then decided to build a 6.6K:8Ω 3 section spec OT from an old 1950's PT with EL75 lams, sounds great in the JCA22H, also changed out the EL84's for EL83's, just need to adjust bias trim pot and jumper a couple of pins the sockets. Picked up some bargain vintage NOS Mullard EL83 for peanuts on the bay.
https://www.ebay.co.uk/itm/333955289351?hash=item4dc1495107:g:O2sAAOSwVCxgcwrX
crazy prices for amperex 6bq5 these days, no doubt.
here are some graphs for a 7.6 K OPT vs the 18 K in the Hohner,
you can see what going through the pentode knee does to the signal,
used this online calc>
https://bmamps.com/ivds.html
here are some graphs for a 7.6 K OPT vs the 18 K in the Hohner,
you can see what going through the pentode knee does to the signal,
used this online calc>
https://bmamps.com/ivds.html
Attachments
Another option that the average self appointed genius bar expert 'tube roller' can't handle is the 15v/300mA heater version PL83, was common in 1950's TVs, I've tried the heaters on a 12v dc wall wart and current draw was about 750mA for the pair, very nice sound when driven hard, got some old used Philips/Mullard ones for free, 1958 date code then bought some vintage NOS Tesla Czechoslovakia with a view to running them cathode starved, tube life may be lower but their humble sacrifice is all for the greater cause.
Attachments
these NOS just came in this morning, another pair on the way.
don't ask.
don't ask.
exactly!
SteveF
Member
Thanks for posting that schematic. Just had a CA-100 come into the shop this AM. It's going to be later in the week before I can get to it, but I'll let you know what happens.
