Bassman Micro (5F6A) in Blackface Bassman Chassis

[King Fan]

Rob'll have to tell us about the 300V on the scheme. As I say I *thought* that was nominal or design max; in his text I quoted above he says we aim for 220 – 300V. Your tube rectifier does drop compared to SS. If I'm right, EZ81 voltage drop is like 25-28V? So if we have 190-0-190 measured, then as Rob says, SS rectified volts might be around 266 or 270, less 28 is ~240.

 

[andrewRneumann]

240VDC out of 190vac is actually pretty good for valve rectifier in my book. You have all the tubes in and running for this measurement correct?

 

[wangdaning]

240VDC out of 190vac is actually pretty good for valve rectifier in my book. You have all the tubes in and running for this measurement correct?

Yes, that is with all tubes. The amp runs fine now, but I had to lower the cathode resistor to get the correct bias. The original schematic has a 300ohm, but that is based on getting 300vdc out of the 190-0-190 PT.

 

[King Fan]

Good. Lemme see if I have the summary straight, for those who come to this later planning EF80 BMMs. You found you could use the 190-0-190 PT, and handle the EZ81 voltage drop, if you decreased the bias cathode resistor to 200 ohms? Remind us what your % dissipation is now?

 

[mountainhick]

Yes, that is with all tubes. The amp runs fine now, but I had to lower the cathode resistor to get the correct bias. The original schematic has a 300ohm, but that is based on getting 300vdc out of the 190-0-190 PT.

Thanks, and sorry for kink I threw into it with questioning the schematic voltages. I am glad it is running fine!

Would love to hear a comparison of a full spec voltage specimen along with yours... Browner sound?

(Also would love to just hear yours!)

 

[wangdaning]

I will try to get a clip of it this weekend, I do not have a cab for it yet so it takes a bit of hooking up. I also realized I wired my bright switch incorrectly. With the 200ohm cathode resistor I am at 100% plate dissipation.

 

[wangdaning]

Take your time guys. I was stressed out this week, my young son said he could not use his leg. He is fine now, and apparently hip synovitis is normal ,but I lost a week with my son saying he could not use a leg. My V3 is now not happy, probably my slaps at it when trying to figure out the above.

To be clear my son is fine, normal thing, need more evidence for that. But he is back and doing his thing.

 

[wangdaning]

After re-wiring to cut out V3 (dirty temp bypass), I now think it is just loose pins in my power section. Remember I did a two channel version, so channel one works without V3. I play a bit and sound drops, one of the EF80s is loosing connection. Seems I will have to buy new plugs and rewire all that, ahh.

 

[Hyakuya]

I've just started testing my Bassman Micro build, EF80 LTP, so I have some voltages to compare with.

My PT puts out too much juice so I fitted a VVR and with that set to give me 300V on B+1 I see B+2 @ 298V and B+3 @ 215V. I'm using an EZ81 rectifier.

Dissipation on the EF80s comes out between 92% and 94.5% with the standard 300R cathode resistor.

 

[King Fan]

Good info. Thanks. I just quoted this report in my thread looking for B+/bias info for these amps, but I’ll hold you to your offer to update there when you have a full voltage report.

 

[wangdaning]

I have ordered some new tube sockets, hopefully over the next week's national holiday here I can re-wire v3 and the power tubes. My son is still suffering from synovitis and can only walk sometimes. He now is out of school for a week and just has to rest. Make matters worse, in the middle of typing this I helped my wife put a pizza in the oven and burnt three of my fingertips on a pizza stone. Probably not playing for a while now. Things will improve at some point, but dang.

 

[wangdaning]

Well, the chassis has been running alright on this for a while now. I am thinking about jumping the voltages as the PT has 270-0-270 secondaries as well, but then I would have to drop for the EF80s I believe (maybe a zener chain). My biggest setback on this whole project has been getting a cabinet though. I paid a guy to make one, with very detailed blueprints, and he failed. Been planning a while now to do what we do, do it yourself. My birthday was half a year ago, but my present came now and I have summer break as well. Hopefully, over the next month, I can get comfy with the new saw and get some cabinets flowing out.