Submini 6112 preamp preliminary build report

bancika · October 04, 2009, 06:32:52 PM

Those miniature boards I posted before (http://www.diystompboxes.com/smfforum/index.php?topic=78925) finally got their home. I got some time to finish the preamp and fire it up this weekend.
I started off with dual channel 6021 based preamp schematic posted by Jeroen on this forum.

To be able to play with higher voltages I built a proper power supply with back-to-back transformer config. I used 12VAC 800mA wall wart and a 110:12V radio shack transformer as step-up.
With 6021 tubes it sounded very very clean. It's like having clean and "slightly pushed" channels, so I swapped both tubes for 6112. Now I'm getting nice crunch on the second channel. Still have to play with resistor values to tweak it to my liking. I think I'll bias the first two stages hotter, it's awkward to have the first stage biased cold. I'll try to bias the third stage colder to clip harder.
Luckily I used those submini tube sockets so I'm able to play with different tubes. On the other hand they are a b*tch to work with, it's not easy to stick 8 very flexible pins in at the same time. It took some practice and nerves to install the tubes.

I wasn't going to go over 80V B+ so I used 63V caps for plate caps. What I noticed is that it sounded better with higher voltages so I'll probably use higher rating caps there and increase B+ to 100-120V (this PS provides up to 130VDC).

For heaters I used heavily filtered and regulated 12.6VDC and wired heaters in series. To get 12.6V I strapped a diode from middle pin of 7812 regulator to ground. Filter caps are 2200uF. I still need to wire LED, prolly gonna use bicolor red-green for two channels.
When I'm happy with the results I'll draw a schematic and post it.

Unfortunately I didn't have my camera while populating boards so I don't have pics of the process.

I'll post updates...

Cheers,
Bane

Skruffyhound · October 04, 2009, 06:45:50 PM

Looking very nice indeed.

frequencycentral · October 04, 2009, 06:48:04 PM

Yeah, very nice! As it happens I've got a nice big batch of 6112 arriving sometime this week.............wondering what to do with them!

Brymus · October 04, 2009, 06:56:22 PM

Very nice indeed,cant wait to hear it.

bancika · October 04, 2009, 08:00:16 PM

tnx guys, here's another one. I'll try to take some better pics next weekend.

Ripthorn · October 05, 2009, 10:28:13 AM

Looking good. I did a preamp based on the same schem, but with a mix of 6111 and 6021. My B+ is 105V currently (looking to get it to ~150V). I get up to a very nice classic high-ish gain sound. I added an extra, switchable gain stage as a boost and it gets all kinds of gain. I look forward to clips, as I have a pair of 6112's just waiting to be used in something (though I am thinking TW Express preamp with switchable parallel 1st gain stage).

bancika · October 05, 2009, 10:53:57 AM

that's interesting. Maybe additional gain stage helps, but with 6021 I couldn't get it past very mild overdrive. I double and triple checked component values and nothing...with 6111 it would probably be even cleaner, since it has even less gain. I should have added a space for additional stage, I have some 5744 tubes which are single triode with 70 mu...

Cheers,
Bane

Skruffyhound · October 05, 2009, 04:57:45 PM

Ooops, now I've got a batch of 6112's incoming. Damn that Ebay.

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Submini 6112 preamp preliminary build report