Bruno Underground 30

[Roe]

I got an Underground 30 in my workshop that needs to get its reverb fixed. It looks like a 6g15 reverb into a top boost ac30 at first look. I will take some measurements of voltages etc

[Colossal]

Thanks Roe. I would be interested to know what the EL84 plate, screen, and cathode voltages look like.

[Roe]

Correction (important!): it is not really the 6g15 reverb as widely reported (see e.g., https://ampgarage.com/forum/viewtopic.php?t=21751). Instead, it is the first vibroking reverb with el84 and the stereo 500k pot: https://www.dagen.no/nyheter/skriftlig- ... os/1487396 (this circuit is based on the 6g15 but differs in some respects)

On the Underground 30 one half of the stereo pot controls the reverb (as on the vibroking), while the other half is the volume pot of the ac30 top boost channel (instead of the fender preamp of the vibroking)

[Roe]

"The Bruno Underground 30 ...offers...a great-sounding three knob reverb that's situated ahead of the first gain stage. This proven combination (think Fender Vibro-King) produces atmospheric AC30 -like shimmer at lower settings" Guitar Player November 97 issue, quoted from http://www.brunoamps.com/rev_5.html

[Roe]

Is there any way to upload a pdf here?

See here for schematic: https://www.academia.edu/144236718/UG30_schematic

[sluckey]

Is there any way to upload a pdf here?

Easy. Look below. May have to scroll down a bit.

[jbrrrrr]

I grabbed it to post here directly - here you go

UG30_schematic.pdf

[Roe]

Thanks Roe. I would be interested to know what the EL84 plate, screen, and cathode voltages look like.

the voltages are on the uploaded schematic. it is like an ac30 except for slightly lower voltages and lower Z (3k3-3k8 probably)

[Colossal]

Thanks Roe. I would be interested to know what the EL84 plate, screen, and cathode voltages look like.

the voltages are on the uploaded schematic. it is like an ac30 except for slightly lower voltages and lower Z (3k3-3k8 probably)

Thank you, Roe. Yes, I saw that. Voltages look similar to a Rocket (quite low).

[Roe]

Thanks Roe. I would be interested to know what the EL84 plate, screen, and cathode voltages look like.

the voltages are on the uploaded schematic. it is like an ac30 except for slightly lower voltages and lower Z (3k3-3k8 probably)

Thank you, Roe. Yes, I saw that. Voltages look similar to a Rocket (quite low).

amp is good for clean tones and modest distortion. my old ac30 will blow it off stage anyday - way more aggression, punch, tightness and better distortion (although it sounds more unforgiving)

[Colossal]

amp is good for clean tones and modest distortion. my old ac30 will blow it off stage anyday - way more aggression, punch, tightness and better distortion (although it sounds more unforgiving)

Interesting.

May I ask what the voltages are in your old AC30? Or what they should be for a JMI 4xEL84? I keep seeing conflicting things: sometimes higher (345VDC B+1) and other times quite low (316VDC B+1).

Many thanks.

[Roe]

amp is good for clean tones and modest distortion. my old ac30 will blow it off stage anyday - way more aggression, punch, tightness and better distortion (although it sounds more unforgiving)

Interesting.

May I ask what the voltages are in your old AC30? Or what they should be for a JMI 4xEL84? I keep seeing conflicting things: sometimes higher (345VDC B+1) and other times quite low (316VDC B+1).

Many thanks.

345v at idle seems right for the late 70s amps. I've lowered the voltages to around 320-5v, using the 245v primary with 230v wall voltage (and a termistor, no standby switch, and ss rectifier)

[Colossal]

345v at idle seems right for the late 70s amps. I've lowered the voltages to around 320-5v, using the 245v primary with 230v wall voltage (and a termistor, no standby switch, and ss rectifier)

Thanks, that is very helpful.

Regarding your rectifier, did you pad the secondaries with some resistance and/or after the rectifier as "sag resistance" to simulate a GZ34? Or just straight SS rectification?

Is this the same amp that you used a CLC in front of B+1 for the whole amp, like the Dave Petersen Brian May amp?

[Roe]

345v at idle seems right for the late 70s amps. I've lowered the voltages to around 320-5v, using the 245v primary with 230v wall voltage (and a termistor, no standby switch, and ss rectifier)

Thanks, that is very helpful.

Regarding your rectifier, did you pad the secondaries with some resistance and/or after the rectifier as "sag resistance" to simulate a GZ34? Or just straight SS rectification?

Is this the same amp that you used a CLC in front of B+1 for the whole amp, like the Dave Petersen Brian May amp?

I have two ac30s with SS rectifiers, no sag resistors, but CLC filters. Mains filtering is 32+32uf on on one and 47+33uf on another. I am using 15nf phase inverter caps. amps are very punchy and aggressive (similar to plexis)

you get away with higher plate voltage if you reduce screen current and voltage. Vox went from using a choke with 420r DC resistance to two 390R resistors in series.

On ac50s and ac100s I'm not using CLC filters, just a small choke between plate and screens. this works well but these amps favor higher filtering (100uf at mains and screens on the ac100, with a 10h choke; 80uf at mains and 47uf at screens of ac50 )