Sano Supernove

[LooseChange]

Take a look at V1A. Do the voltages seem right?

[tubeswell]

I can't seen any voltages written on that schematic (except for some pencilled notes about the output stage?)

(but as an aside, the load for the 12AU7s throughout that amp is way too suboptimal IMHO - would be a lot better if it was closer to 10k-20k per tube)

[LooseChange]

Says right on the print:
Plate: 110v (Rp= 100K)
Cathode: 1v (Rk=2k2)
Thanks!

[tubeswell]

Oops - so it does - sorry I missed that.

The plate voltage looks about right for that setup (assuming 300V HT). 1V on the cathode seems funny tho' -  Here is a load line I did just now for V1 - shows about 93V on the plate and -4V at the cathode, and gives a voltage gain of 8.4! (hi-fi amp)



FWIW 33k plate load makes the load line look better (175 plate volts and -8V bias with Rk = 2k2) for a power amplifier with a voltage gain of 5.7 on V1

[HotBluePlates]

V1a voltages look fine.

I somewhat disagree about the suboptimal loading... Since the input signal at this point is a smallish fraction of 1v, the operating point doesn't need to be centered.

Graphically determining tube constants from the first graph, mu look like 16.5, rp looks like 16k. A = 16.5*[100k/(100k+16k)] = 14. So more gain had from the suboptimal load, and at lower current draw from the power supply.

The voltages for V1b seem weird unless you notice it is a split-load inverter. The 100k cathode load is not in its "obvious" spot.

And this amp has a sorta-differential amp after the split load to make up gain. It's not a terribly good one, lacking the large tail resistor because the supply voltage is fairly low. Regardless, it probably functions well and relies on the split load to provide balance lost by omitting a big tail resistor.

[DummyLoad]

Take a look at V1A. Do the voltages seem right?

No! Vk of V1A is higher than 1V. closer to 4.25 or thereabouts. Rk value of V1A is missing decimal point.

Vk @ 1V with a 22K Rk... not unless that bogey was not a 12AU7 or the B+, Va, or tube type is a misprint.

with a 2.2K Rk and Ibias of 1.9mA with 300V B+, a Va of 110V and Rl of 100K, Vg will be around -4.25V

with a 22K Rk and Ibias of .57mA, with a 300V B+, a Va of 230V and Rl of 100K, Vg will be around -12.7V

mod - DC couple V1A & V1B - drop value of Rk of V1A to 1K. this sets Va of V1A at ~~75VDC - with Vg of V1B set to 1/4 of B+ (~75V) then you have a happy DC coupled cathodyne and gain stage. you then eliminate need of .01uF, 220K and 2.2K resistors of V1B.
 
--DL


EDIT: errr... a bass amp?

[PRR]

I see about 1.5mA missing.

Agree that probably the cathode voltage was noted wrong, 3V to 5V seems more likely for 12AU7 in these conditions. Rk is probably 2 *point* 2 Kay, not 22K. (Make your decimal points loud enuff to survive fax, Zerox, $29 scanners and JPEG gibberish.)

[LooseChange]

Thanks guys!
I am glad you guys helped me out. My problem is that the B+ on this amp is low. I think the PT was changed. It's 410v. V1A reads Plate: 157v, Cathode: 3.9v, I checked the tube and the components all are within spec.

[DummyLoad]

drop a hammond 279X in there. that should get you back up in 500V B+ territory and the output power back up to where it was originally.  

--DL

EDIT: or, go SSR with 6 x 1N4007. that should up B+ to 450V +/-.