Bazzman

[Lynxtrap]

Thanks for the encouragement
The problem with blowing heater resistors is overshadowing everything else at the moment. There must be high voltage dc going to the heaters somehow. I just can't figure out why and where...

It would be great if there where reliable schematics available for that preamp. As far as I know, the dr could be reading this right now, laughing hiss a** off

[tictac]

If you do have a center-tapped heater winding as shown in your schematic you don't need the 100 ohm resistors...

Just ground the center-tap and wire up your heaters...

Another thing to check is make sure each lead coming off the heater winding is going to the same point throughout the circuit...

I.E. one wire going to all pin 2 (power tubes) and pin 9 (preamp) the other wire going to all pin 7 (power tubes) and pin 4,5 (preamp). Using two diffrent colors of wire help when doing this...

TT

[Lynxtrap]

If you do have a center-tapped heater winding as shown in your schematic you don't need the 100 ohm resistors...

Just ground the center-tap and wire up your heaters...

Sorry, I haven't updated the schematic yet. There is no centertap.

Another thing to check is make sure each lead coming off the heater winding is going to the same point throughout the circuit...

I.E. one wire going to all pin 2 (power tubes) and pin 9 (preamp) the other wire going to all pin 7 (power tubes) and pin 4,5 (preamp). Using two diffrent colors of wire help when doing this...
TT

Well, that might do something, maybe less hum. It's something to try as sort of icing on the cake in this case. or if I rewire the heater circuit.

Those resistors go up in smoke when the amp gets full B+ when the tubes are in. The resistors are a symptom of something worse. A bad tube could be one.[/quote]

[timford]

Finally had a go at some ef86 load lines (it's been over a month). Not sure if it is of much use to you at this stage, but it was a good learning experience (and I'm not even sure how correct they are). Spent a lot of time measuring voltages and doing lots of sums and scratching my head. Surprised how far under the knee all of these are. Come across a lot of ideas for things I need to experiment with.

They have come up in the opposite order to what I intended, so start at the bottom and work up.

Measured the tone pots and they are both linear 1M.

Hope the project is back on track. Spent a lot of time looking at the c30 PI dual inputs and will be interested to compare back to what you are doing.

Check your PMs

[Lynxtrap]

Thanks a lot, Timford! Great work!
I haven't been around here for a while, and I haven't done anything to the amp in over a week.

I did try putting in one tube at a time while measuring heater voltage at each socket. Everything looked fine. I just made one mistake: I did all this in standby.
As soon as I turned on B+, the second heater resistor instantly went up in smoke.
I don't have any spares, so I need to order some parts. I'll also order some 1 Meg pots for the tonestack.

I'll have a look at those schematics when I get them, that's really kind of you!

Of course, I'll make updates as soon as I get to work on the thing again. Unfortunately I won't be able to do any advanced calculations, since I don't have the knowledge yet.

[Lynxtrap]

Finally, there is sound and no smoke (yet) from the amp.
It's late in the evening, the wife is sleeping, so I've only tested it at bedroom level so far. Not actually the bedroom, but anyway...

It seems very promising. I'm not sure where the "z" preamp got life from, as it was weak sounding before. But I now have a 220K resistor to the anode compared to 100K before, and 1 meg tone controls instead of 250K. Those things might have done it.

If this amp had a name, it would be merely "Bazzman", no "66". It will run on 6L6GC, mostly because the KT66 sit too close to each other for comfort.
Got a new pair of Tung-Sols running at 16,5 watts at the moment.
I'm measuring 495-500 volts at idle, it's a bit more than I'd like.
But I'm not sure what to do about it.

Until I get a new schematic done, this is what I've got:

- Power line is close to stock Bassman Ten, including filter cap and resistor values, plus choke. Solid state rectification
- Bias line has both voltage and balance adjustment but is otherwise stock Bassman.
- One preamp is pretty much a Fender blackface 'normal' channel as on the schematic above.
- One preamp is (presumably) pretty much a R66/z28 as on the schematic.
- The PI is also like on the schematic + a DC30-style master volume (which probably won't be a keeper). No negative feedback.

Even on these low levels, the amp seems to distort easily. I don't mind, but it seems funny.
There is also a hum that does not go away by balancing the output tubes. Probably lead dress, I hope... I'm still a bit worried about the heaters, got better resistors but no fuse. Everything seems normal though, except for the hum.
I'm sure it will blow up tomorrow

[Lynxtrap]

Edited the topic to say more about the content...
The amp seems to be working fine. The Z-channel is a dream.
The F-channel breaks up very early with the master down. Funny. I haven't done much to it, except for made separate cathode connections for the triodes. It shouldn be this aggressive, but maybe it's the PI distorting with the master down.
The amp really begs for hotrodded country licks and gnarly Tweed bassman-type riffs

[Lynxtrap]

I finally got the schematic drawn. This is the amp at the moment (unless I made any mistake drawing the thing).
Feel free to comment!


[img726]http://i1181.photobucket.com/albums/x43 ... 3-webb.jpg[/img]

[Lynxtrap]

I've noticed some things so far. They might be problems or not.

- The anode voltages are high overall. 500V at the output tubes, 300V at the PI, 270V at the AX12 and 116V at the EF86.

- The "F"-channel does distort more and sooner. Why is that..?

- The whole amp is too bright. Maybe a cut control before the master would do it?

- The master vol pot should not be linear.

[timford]

With the ef86 front end, maybe try the 66 caps in the tone stack (1000pF instead of 250 and .01uF instead of .022). I'd be interested to do the specific math, but the 66 is a much less trebly amp than either the z28 or kt45.

But as both channels are bright....... I don't have those answers at my fingertips yet so I'll go and consult Mr Kuehnel and do some power amp analysis....... I'm sure someone will chip in with the perfect answer off the top of their head before I get back to you though

Best of luck. I admire the way you are choosing to learn.

[Lynxtrap]

Thanks timford! It's a learning process for sure

It would be fun to try the other cap values! The tonestack is the key to that channel, IMO.

I'm quite happy with the EF86-channel overall. The treblyness is also prominent with the F-channel, so I figured it's a "global" thing that has to do with the PI.

What are your opinions about a 100pf across the PI plates, like the 66 seems to have? Or would a cut control be the same but more flexible?

[ChrisM]

Z channel remove the 250pF cap, if that's not enough remove the 0.0068uF cap too.

F channel remove 0.015 cap and replace it with a short. It's not needed.

[ChrisM]

If you want a cut control add a 0.0022uF - 0.0047uF cap in series with the MV pot.

[Lynxtrap]

Thanks for the advice!

I've added a cut control, a 250k pot in series with a .0022 cap. It didn't do much really, just takes away some of the highest precence. I should've gone a bit higher with the cap, maybe .0047, but I don't have any at home.

I thought I'd try 1000pf instead of the 250pf in the preamp. Found an old 1200pf in the drawer. Since I have a spare switch in the Z-channel, I decided to have the caps switchable.
It's no major difference, but with the 1200pf there is slightly more punch or body in the mids.
That channel stays like that now. For a while

I removed the .015 cap in the F-channel tonestack. Thanks for that ChrisM. I had completely forgot about it.
The original Bassman Ten had a .0047 there. Then I swapped it for a .015 and it just stayed there. But it does not belong in a ab763 tonestack.

I also noticed that the .047 before the mid pot was actually .022, and corrected that. That got rid of some boom in the bottom/low mids.

Then I changed the coupling cap from the F-channel to the PI. It's now .01 like in the original amp.

All in all, it's a very different amp in many ways compared to when I started this project.
Still, it stills sound very much like a Fender.

What surprises me most is the loss of headroom. I can't explain it, but I guess it has to do with the post PI master. The PI obviously starts clipping very easily. But I'm not complaining.

[Lynxtrap]

Here is the updated schematic in case anyone is interested.
I'm actually running 12AT7 in both the PI and the F-channel at the moment.
The cut control is much better with a .0047 cap.
Feel free to comment

[IMG736]http://i1181.photobucket.com/albums/x43 ... 4_webb.jpg[/img]