Brownface Super Bias circuit diode

[pompeiisneaks]

It's also much safer and meets most requirements by regulation to not put the earth to a transformer lug. You should instead drill a new hole and add a screw directly through the chassis. Transformers due to weight and vibration, can cause the lead to come loose over time, losing the safety earthing.

~Phil

[brwnfce]

Thanks, and I hear what you are saying, but I'm not going to be drilling any holes in the amp, plus that was the site of the original earthing/death cap,now removed, the connection is soldered ,crimped, shrink tubed and taped, well tightened down. I'll be checking it over regularly and it won't be getting moved around in the normal course of events.
Cheers.

[brwnfce]

I've sorted the power cord and secured the earth bond with some loctite thread locker adhesive. Amp sounds good though the Harmonic Tremolo is a little underwhelming, weak, has to be at max intensity and volume around 6/7 to really hear it at all, I've subbed in several known good strong tubes in V3, not hearing any improvement and it sounds like a standard bias trem to me not hearing that phasey effect often refered to, time to change the ceramic caps in the oscillator circuit and/or other components?

Vibrato not actually working at all now.

[martin manning]

I've sorted the power cord and secured the earth bond with some loctite thread locker adhesive. Amp sounds good though the Harmonic Tremolo is a little underwhelming, weak, has to be at max intensity and volume around 6/7 to really hear it at all, I've subbed in several known good strong tubes in V3, not hearing any improvement and it sounds like a standard bias trem to me not hearing that phasey effect often refered to, time to change the ceramic caps in the oscillator circuit and/or other components?

Vibrato not actually working at all now.

Get voltages on plates, grids, and cathodes for all of the triodes in the vibrato (V3, V4, and V5) with it switched off and post them.

Failure of the caps in the LFO's feedback loop (0.02 and 2x 0.01) is common, so I'd start by replacing those. Check the values of all the resistors in the vibrato to see if they are still within tolerance. If it's still not working satisfactorily once those are verified, try checking or substituting for the other caps. Tubular film caps are much more likely to leak than ceramics.

[brwnfce]

Hi Martin,

Many thanks for your response, measured resistor values are in red in attachment, voltage readings are as follows as per your recommendation,

V3 triode A
plate 140 vdc
grid 0 vdc
cathode 1.5 vdc

V3 triode B
plate 295 vdc
grid 140 vdc
cathode 1.5 vdc

V4 triode A
plate 0 vdc
grid 0 vdc
cathode 0 vdc

V4 triode B
plate 258 vdc
grid 20 vdc
cathode 56 vdc

V5 triode A
plate 195 vdc
grid 0 vdc
cathode 0.2 vdc

V5 triode B
plate 195 vdc
grid 0 vdc
cathode 0 vdc

[brwnfce]

I'm awaiting delivery of replacement components right now, not changed anything out as yet.

[martin manning]

V3 triode A
plate 140 vdc
grid 0 vdc
cathode 1.5 vdc


V3 triode B
plate 295 vdc
grid 140 vdc
cathode 1.5 vdc

These voltages are low. The plate of V3B should be ~350V per the schematic.

V4 triode A
plate 0 vdc
grid 0 vdc
cathode 0 vdc

V4 triode B
plate 258 vdc
grid 20 vdc
cathode 56 vdc

Again lower than the schematic.

V5 triode A
plate 195 vdc
grid 0 vdc
cathode 0.2 vdc

V5 triode B
plate 195 vdc
grid 0 vdc
cathode 0 vdc

Something is wrong here. These two should have a common cathode voltage ~3.5V as they share a 4k7 cathode resistor and 4u7 bypass cap. Looks like there is excessive current draw, which would be the case if the cathode is shorted to ground, and this may be dragging the other voltages down. Check the resistance across the 4k7/4u7 and see if it is around 4k7.

[martin manning]

...Oops

[brwnfce]

I'm getting 4.64k there. And having checked again the cathode is exactly 3.5vdc. I'm running the amp at exactly 110 vac, getting 432vdc on the 5881 plates.
Going right to left in the images the caps read as follows measured in circuit without lifting leg.

ASTRONS

.05 outer foil= 60v, 300v
.02 outer foil= 167v, 0v
.05 outer foil= 248v, 0v
.03 outer foil= 0v, 170v
0.1 outer foil= 0v, 0.003v
.05 outer foil=62v, 0.03v
.05 outer foil=292v, 0.08v
0.1 outer foil=10v, 20v

BLUE MOLDED

0.1 outer foil=70v, 0.001v
0.1 outer foil= 167v, 0.003v

SOZO's

0.1 outer foil=0v, 63v
0.1 outer foil=0v, 293v

0.1 outer foil=290v, -50v
0.1 outer foil= 297v, -50v

IMG_1225 (1).jpeg

IMG_1226 (1).jpeg

IMG_E1227.jpeg

[martin manning]

I'm getting 4.64k there. And having checked again the cathode is exactly 3.5vdc. I'm running the amp at exactly 110 vac, getting 432vdc on the 5881 plates.
IMG_1225 (1).jpegIMG_1226 (1).jpegIMG_E1227.jpeg

110VAC is on the low side for the AC, so that probably explains the reduced voltages with respect to the schematic. 115-117VAC would be typical for 1960's US line voltage.

The DC voltages you measured on the grid side of the caps look ok except for the Astron's with 30 and 80 mV. Those seem to be leaky.

What are the DC voltages between the ceramic caps in the oscillator (vibrato off)?

[brwnfce]

The amp label asks for 110v, schematic states voltages +/- 20%

IMG_1238.jpeg

Measured from chassis ground to the point marked by red dot, 172 VDC, from dot to 1st capacitor leg, 124 VDC, from 1st leg to 2nd leg, 0.973 VDC, from 2nd leg to 3rd leg, 1.3 VDC

IMG_1239.jpeg

Measured from chassis ground to the point marked by red dot, 0.006 VDC

IMG_1239.jpeg

Measured from dot to dot, 60 VDC/-60 VDC

IMG_1239.jpeg

Also the Astrons seem like they really are on the way out, in addition to the 2 you identified as leaky, two of the other 0.05uf are reading around a volt on the grid side now, I'm tempted to just change them all out.

[martin manning]

What are the voltages here?

[brwnfce]

with the - probe on the chassis, first ? I get 170VDC, 2nd ? I get it switching between 27 VDC and -OL

[martin manning]

with the - probe on the chassis, first ? I get 170VDC, 2nd ? I get it switching between 27 VDC and -OL

With the oscillator off you should be seeing zero volts at the lower ?. If you pull the oscillator tube out and and switch the oscillator on (footswitch lead not grounded), you should see 0V at both ?.

[brwnfce]

with oscillator tube out, lower ?= 0.078 VDC, upper ?= 0.007 VDC