B-18 PI Question

[jon]

I built a B-18 copy and I am enjoying it a lot. One thing that keeps haunting me is the PI this amp and others from Ampeg in the same period uses a Triode/Pentode PI arrangement. The pentode is a gain stage feeding a split load triode stage. The down side to this arrangement is that the pentode distorts easily. This is not what I am looking for in a home use bass amp.

I have been thinking of reworking the PI to something else. My original thought was a 12DW7 with the AX side acting as the Gain stage and the AU side swinging lots of voltage to drive my 6L6/7027 or 6550 output tubes. I would like to keep the basic archetichure of the amp the same but just modify the PI. Any thoughts?

BTW space is an issue in this amp. Meaning I don't have any.

[Phil_S]

If you are going to add a gain stage with the X side of the 12DW7, consider using the U side as a concertina. Although the concertina has no gain to it, the added gain from the X side driving it should compensate adequately. This would greatly simplify the circuit and is, IMO, the perfect use of a 12DW7.

[jelle]

Dude, bring it over and we'll scope it. Mod it and scope it again. I should have some nice tricks for this one.

[jon]

OK OK OK

[jelle]

[Bob-I]

Distort? I played bass with a B18 back in the 60's. With. P bass I could run it almost wide open and it was clean. I also used a b15 along with the 18 for some gigs, awesome tone and more power then I could use at the time.

[KT66]

I agree with Bob, should be a clean machine. Have you tried subbing another 7199 ? Is it the stock circuit with both channels ? Maybe if you posted a pic one of us could spot something you missed.

[jon]

As requested Here is a pic.

[KT66]

Jon, nice looking build - I like the layout too, It's Ampegish in a way, did you design it ?

Anyway, I think I found the problem - There is a white wire that is meant to connect the screens of the output tubes, but on the tube on the left it is connected to the control grid ( pin 5) instead of pin 4.

Also, most amp designs that you see past like 1970 usually have 470R screen grid resistors for 6L6/ 7027 types, if you are running typical plate voltages I would think about putting them in. Andy has posted some interesting stuff on screens in this section also if you wanted to get a little fancier than that.

[jon]

Actually, the wire is bent at a 90 and is hidden under the screen feed. It is located on the correct pin.

[Structo]

What are the power tube plate and screen voltages?

[jon]

When I get home on Wed I will pull my spread sheets and let you know.

BTW I found something that may explain something. The original amps show the cathode of the pentode gain stage going through a 1500ohm resistor and a .05 cap to ground. I eliminated the cap and ran the resistor to ground. This couldn't be the source of my distortion, could it.

[KT66]

I guess my eyes aren't so hot anymore, sorry about that. I scrutinized your pic and checked off every connection on the Joe Piazza schematic and it all looks good to me, although I couldn't make out some of the tone stack connections because of the angle, but again nice job. The only difference I noticed compared to the schem was the mica snubber cap that you added between the pentode plate of the 7199 and ground, but I wouldn't think that would be the cause of distortion. I also couldn't make out the bands on the resistor that connects pins 1 & 9 in the PI, which should be brown, black, blue for 10M.

The .05U cap that you mention above should have been left out. The way fliptops are wired is such that when the speaker cable is unplugged, that cathode is lifted from DC ground and no signal passes from that point on, which prevents there being a signal on the OT secondary with no load attached. When the speaker cable is plugged in, pins 2 & 3 are jumpered together, the cathode resistor is connected to DC ground, and the .05U cap is out of the circuit.

[tubeswell]

Hi jon, are you using 6SL7 pre-amp tubes?

[jon]

I guess my eyes aren't so hot anymore, sorry about that. I scrutinized your pic and checked off every connection on the Joe Piazza schematic and it all looks good to me, although I couldn't make out some of the tone stack connections because of the angle, but again nice job. The only difference I noticed compared to the schem was the mica snubber cap that you added between the pentode plate of the 7199 and ground, but I wouldn't think that would be the cause of distortion. I also couldn't make out the bands on the resistor that connects pins 1 & 9 in the PI, which should be brown, black, blue for 10M.

The .05U cap that you mention above should have been left out. The way fliptops are wired is such that when the speaker cable is unplugged, that cathode is lifted from DC ground and no signal passes from that point on, which prevents there being a signal on the OT secondary with no load attached. When the speaker cable is plugged in, pins 2 & 3 are jumpered together, the cathode resistor is connected to DC ground, and the .05U cap is out of the circuit.

Yeah that is a 10m.

and yes i am using 6sl7gt tubes.