7 watt 6BM8 amp w/VVR

[10thTx]

This is the second 6BM8 push/pull amp I've built. Originally this amp had EL84's but I changed those to 6BM8 pentodes. It's called a 56T reverb and is based on an amp drawn up by DaGeezer on another forum.

The amp is about 7 watts. I play with a band where we mike it into the P/A and it works fine for me.

I added a VVR3 to it so I can get even more overdrive at very low volumes. It's amazing to me that I can get a decent overdrive tone at about TV volume levels. NO effects pedals were used. Just guitar into amp and some delay & EQ in the mixing.

I have a sound bit (if it tries to close just hit "no" & it should play). Excuse the sloppy mediocre playing, ... the purpose is to hear the amp's overdrive (perhaps at a 3-4 watt level). I used a backing track I found.

http://www.soundclick.com/player/single ... ID=7988948

The amp was played at a very low volume so I could easily hear the backing track played thru very tiny cheapo computer speakers which were maybe at 2/3 volume.

The VVR came from here:
http://hallamplification.com/main.html?src=%2F#2,2

I used it on on the power amp and the LTPI vs. just power amp or entire amp. I've posted a schematic to show what I did. I haven't seen much in schematics showing the VVR on both the power and phase invertor.

With respect, 10thtx

[Phil_S]

Great sound. Playing is respectable, too. No need to feel bashful about it.

Lotta amp for low watts, though. LOL, it matters little how many watts you want, it's still pretty much the same number of tubes and parts! Nice job. Got some gut shots to share?

[10thTx]

The inside of the chassis has gotten to be a mess now. I built it without reverb, without effects loop and without VVR and then went back and added it all to the tiny chassis. The amp has had several evolutions to become this one including an additional 12AX7 tube added.


Here is the cabinet:
http://s28.photobucket.com/albums/c216/ ... mbopic.jpg

This is what looked like originally:
http://s28.photobucket.com/albums/c216/ ... rbtank.jpg

Even though the wiring is messy, it's very quiet at idle. Cab is about the size of a 5E3 perhaps?

With respect, 10th

[Structo]

Beautiful cab on that.

[IMG:538:404]http://i28.photobucket.com/albums/c216/ ... mbopic.jpg[/img]

Build is also respectable.

[IMG:800:600]http://i28.photobucket.com/albums/c216/ ... rbtank.jpg[/img]

[tubeswell]

I couldn't get the soundclick link to work (even tho' I have been a member for some time - go figure) but that VVR looks like a neat idea. How much of a voltage reduction can you get out of it?

[10thTx]

You know I didn't measure the lowest it could go. It did drop the voltage on 6BM8 pentode plates by over half at a setting that I thought sounded particularly sweet.

The soundclick linked worked on both my computers? A pop up would appear that stated did I want the window closed ....... and I clicked "no".

With respect, 10thtx

[pullshocks]

I had trouble getting to his file when using Firefox (it just takes you to the soundclick home page). Internet explorer works like he sez.

[10thTx]

Here's another sound bit done with a backing track:

http://www.soundclick.com/player/single ... ID=7971197

No pedals just EQ and a some delay.

With respect, 10thtx

[Gee]

I was interested to see how you scaled the PA and PI. I just did a similar thing on an 18 Watt. I see you have a third diode after the VVR. Why did you add the third one after the VVR? I wasn't aware that one is needed here.

Also you have a redundant 220k bleeder resistor on the C node which will double the current draw to C.

[10thTx]

The extra diode did not seem needed to me either. And I've never used bleeding resistors before.

I simply did it that way because a guy who has done a number of them recommended it that way. I think I'll leave the diode after the VVR cause I don't see that it will hurt anything and remove the extra 220k resistor on node C.

Thanks for your comments. With respect, 10thtx

[Gee]

The extra diode did not seem needed to me either. And I've never used bleeding resistors before.

I simply did it that way because a guy who has done a number of them recommended it that way. I think I'll leave the diode after the VVR cause I don't see that it will hurt anything and remove the extra 220k resistor on node C.

Thanks for your comments. With respect, 10thtx

I searched around a bit and found an early schematic of the VVR that showed a diode after the VVR as an option to be used when not scaling the whole amp. However, I think it is redundant when you use 2 diodes before the VVR to isolate the scaled and non scaled circuits.

Those 220K bleed resistors are just there for safety so that the caps drain away when switched off. Only one per bank of connected caps is required.

[Structo]

Nice smooth sounding amp.