KT66 JTM45 / JMP50 hybrid?

[RockinRocket]

yes its not a 18 watter but it look so close to one I keep confusing myself Thanks for posting!

[ezietsman]

I got as far as this today, maybe I can fire it up in the morning



The flyleads off the board are not soldered to the chassis yet, I'll do that in the morning, that's all thats still needed.

[ezietsman]

It's done. Last wire soldered on Star Wars Day! Yay!

I haven't switched it on yet, went for dinner, had beer. I'll do the first startup tests and biasing tomorrow after coffee.

[ezietsman]

Meh, my choke shorts to its casing, I'll have to get a new one.

[TWANGGG]

Nice work, I've enjoyed following your progress.

Sorry for your problems with the choke. I wonder if they test them at all?

[ezietsman]

Nice work, I've enjoyed following your progress.

Sorry for your problems with the choke. I wonder if they test them at all?

Ja this sucks, also, I live in South Africa, so I can't easily get a replacement sent to me (for reasonable money). I guess I'll try and get some more fuses tomorrow and a 120Ω 5 watt resistor so I can find the rest of the problems.

[ezietsman]

OK so some progress. I have all the tubes in and the amp is running into the dummy load. No fuses blowing. I haven't checked whether it is making a sound nor have I tried to bias it. I'm getting correct voltages as one would expect everywhere except for in the preamp tubes (V1) where my plate voltages are very high (±270V). I don't see a wiring mistake anywhere. I have a 1k resistor in place of the choke for the moment and the amp follows the 50 watter schematic (mostly) except for the power section which looks like a JTM45 (mostly). The power rail is that of a 50 watter. I'm getting 475V B+ on diodes and 450V B+ on the GZ34.

Common wiring mistake?

[ivan H]

Hi, I haven't been here for a while so its good to see the progress you have made. Its looking good. I like the split/shared cathode switching. With the SS/tube rectifier switching, you may have to adjust the bias supply dropping resistor's value on the switch to suit your power tubes. I temporarily used a pot to work out the value needed. My amp has a Marstran 1202-55 PT & the voltages at the rectifiers are pretty much bang on what you have there. I imagine you currently have the bias set to the largest negative voltage at the moment so the power tubes will be drawing very little current. Your HT voltage will come down some when you bias them up. Cheers

Edit, bias it up with it switched to SS rectifier mode first. I did that, then attached the pot across the switch, set to 0 ohms, then slowly dialled it up til I had identical power tube current draw (tube rectifier mode) as I had in SS mode. Took the pot off & read its value. Cheers

[ezietsman]

Hi, I haven't been here for a while so its good to see the progress you have made. Its looking good. I like the split/shared cathode switching. With the SS/tube rectifier switching, you may have to adjust the bias supply dropping resistor's value on the switch to suit your power tubes. I temporarily used a pot to work out the value needed. My amp has a Marstran 1202-55 PT & the voltages at the rectifiers are pretty much bang on what you have there. I imagine you currently have the bias set to the largest negative voltage at the moment so the power tubes will be drawing very little current. Your HT voltage will come down some when you bias them up. Cheers

Edit, bias it up with it switched to SS rectifier mode first. I did that, then attached the pot across the switch, set to 0 ohms, then slowly dialled it up til I had identical power tube current draw (tube rectifier mode) as I had in SS mode. Took the pot off & read its value. Cheers

Yes, I had to change the bias supply's resistor, I'm using a 220k and a 1M in parallel now, perfect (didn't have a 180k). My two power tubes aren't perfectly matched but I'm getting around the same values on both SS and valve rect modes, biased just under 70% on both. I'll fine tune it once the valves have settled down.

I had to remove the choke for now, there's a 1K resistor for the moment. I have quite a bit of hiss when the volumes are dimed, sounds like a dry joint and its probably that one connection that is connected underneath the board, so I'll have to remove it, again! if I remove V2 the amp is absolutely quiet and if I remove V1 it still hisses. So I know where to look. I'll chopstick it some more today.

It sounds pretty good so far, I'm quite happy.

[ezietsman]

Some more reflections after playing it a bit. I get massive hum when I patch the two channels. It may be because my patch cables are cheap and are picking up the transformer's hum. I'll try with better cables. Also, I get some hum, sounds like 60Hz, unless I dime the normal volume, at which point it goes away. Strange.

[neikeel]

Some more reflections after playing it a bit. I get massive hum when I patch the two channels. It may be because my patch cables are cheap and are picking up the transformer's hum. I'll try with better cables. Also, I get some hum, sounds like 60Hz, unless I dime the normal volume, at which point it goes away. Strange.

The correct choke 'resistor' would be a 100r Ercol of about 10watts rating.

If you get a hum when bridging (don't usually bridge myself) on a new build it is typically an earthing problem with one of your switching jacks - a cheap cable will not do that unless it is actually broken or not sitting in the socket properly.

[ezietsman]

The correct choke 'resistor' would be a 100r Ercol of about 10watts rating.

I have a 100Ω 5 watt also but I'll keep the 1k in for now, until I can get the choke sorted out.

If you get a hum when bridging (don't usually bridge myself) on a new build it is typically an earthing problem with one of your switching jacks - a cheap cable will not do that unless it is actually broken or not sitting in the socket properly.

Thanks, this narrows it down even more. I'll have a look later.

[neikeel]

Oh and some of those long wires in the preamp to the switches are asking for noise (crosstalk) and PO.

[ezietsman]

Some sound clips.

https://soundcloud.com/ewald-zietsman/ez68-demo-1

Detailed description on soundcloud. TL;DR: Amp into dummy load, speaker impulses, Tokai Love Rock, Played same part 3 times: JMP50 lead mode, JMP50 Bass mode and JTM45 Mode. Amp was otherwise left alone between takes.

[ezietsman]

...and here's the amp inside its shell