• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

old console amp

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It's got enough *** to get decent sound out of some sony bookshelf speakers. Bass response is kinda weak, but the mids and highs are really listenable. I like what I'm hearing enough to want to seriously take a good look at something more substantial at some point- a Dynaco or something. I'm just gonna use this one as a "workshop" amp with the ipod. I spend a good deal of time in the shop so it will get used so I'd like to make it safe and reliable. It will make a good, functional, conversation piece. No one I know has a tube amp other than for a guitar.
 
frugal-phile™
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The grey stuff can be removed, the tone controls & the filter on the front end for a ceramic input (Eli correct me if i'm mistaken)

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The way the 12ax7 gets its B2+ from the output screens is strange to me. I'd rejig that to get higher voltage and maybe subbing in a lower mu tube (ie change the front end completely).

I'd also remove the (R12, R13, C12) & (R17, R15, C13) networks. Some sort of HF rolloff?

Balance control could go as well.

dave
 

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I'm surprised. It looks like a silicon diode, not selenium, is used as the B+ rectifier.

The console certainly used a piezoelectric cartridge. I'm thinking crystal, not ceramic, as the higher signal voltage produced would be needed to deal with the tone control losses.

The photo of the schematic is cropped. Please post the portion that shows how the B+ PSU and heater string are put together.

Dave is right on the money about simplifying things. I'd go whole hog and replace everything, except the "iron", tubes, and chassis. Guys, I'm thinking a pair of inexpensive Alpha Taiwan controls (1/channel) for volume and balance, possibly CCS loading the voltage amplifier, and "plate to plate" partial NFB. I'm very leery about putting those O/P trafos inside a GNFB loop. How much bass bandwidth limiting would be necessary to prevent core saturation? Oh yeah, regulating 50EH5 g2 B+ would be a nice touch.
 
You'd think silicon diodes were made of solid gold in those days. They could at least use an extra three and get full wave.

ABSOLUTELY NOT, as it's a transformerless AC/DC model. If you directly full wave bridge rectify the AC mains, both the + and - sides of the resulting rail are deadly dangerous. There is hope for the chassis being "cold", when 1/2 wave rectification is employed.

The discussion has included remarks about the NECESSITY of adding an isolation trafo. Once an isolation trafo is added, full wave bridge rectification is required to protect the power trafo from the "standing" DC 1/2 wave rectification produces.
 
In my next 50EH5 build i'm going to try no feedback pentode again and use a set of speakers that like current amplifiers.

dave

The OP tried the amp with some conventional speakers and noticed mediocre bass. While nothing can be done about the bandwidth limitations of the O/P "iron", the so called plate to plate connection for short loop NFB will improve damping factor and (hopefully) yield better bass performance. I'm thinking CCS load the voltage amplifier to make up for the reduced gain of the O/P stage.
 
The bass is not just terrible, but it does seem to roll off a lot sooner than what I know the speakers to be capable of. I was not aware I would need to go full wave after adding the iso. Guess I need to study and get up to speed on this thing- however, it appears this little amp is a bit unconventional. I need to order that tube amp book I have seen mentioned here. I want to get it to a point where I can operate it safely- then do a few minor parts upgrades- then just listen to it for a while before I tear it apart and do anything radical. Really, I think I'd rather just leave this one as is and get something larger and more conventional to play around with.
 
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OK, I think I've come up with a reasonable solution to the safety/PSU issues. It's uploaded. Guys, please check me out.

Better energy storage may help improve bass performance. All original electrolytic caps. need to be replaced, as they physically dry out over time. The space currently occupied by the triple section cap. should certainly prove adequate to hold a 220 μF./250 WVDC part. A clamp will be needed. Modern 'lytics cram a good deal more capacitance in per unit volume.
 

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