Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Redfishbum on June 09, 2016, 10:34:13 am
-
Greetings.
I picked up a early 60's tube powered Westinghouse record player for cheap thinking that I might be able to turn into a Cigar Box tube amp. It looked a little "weird" to me with only 2 tubes and thought it might have a diode rectifier. Nope...I found a copy of the schematic. THere is no power transformer and no fuse. SCARRY STUFF. It has a half wave rectifier, 50DC4, and an output tube, 50EH5.
If I were to try to do this, I'm thinking I would need to put in a 3 prong power cable, use two small power transformers on the front end for isolation. However, it will get bigger because of the transformers and not sure it's worth the effort.
Thoughts?
Bart
-
> not sure it's worth the effort.
Agree. It isn't nearly a guitar amp.
You know it is potentially lethal without more stuff.
It does not have enough gain to play guitar well as-is. This was a last-year tube phono using the HIGH!-output phono cartridges developed to eliminate a preamp tube. You need to add a gain of 10-100 in front, or whang very hard for not much output.
I have a Kent in the garage, which even has a preamp, but just is not worth the power upgrade. If my neighbor was into junker amps and had money, I'd do it; but he's a car-nut.
-
A related project. Zenith used to make an AM transmitter that connected to their phonograph. The signal could be picked up by an AM radio in the room. Antique Electronics sells a kit. Now if you could interface a guitar to the transmitter, you could use an AM radio as an amp.
http://webpages.charter.net/aj8mh-radio/phonoscil.html (http://webpages.charter.net/aj8mh-radio/phonoscil.html)
-
Thanks for the replies. I think I'm gonna skip on this death trap circuit and stick with standard circuits that I'm used to dealing with.
Amazing to see what was actually sold to the public in years past.
Bart
-
If you want to use your small amp and keep it small, you can add this stage
(http://www.hawestv.com/amp_projects/amp_solid_tube/art_guitar_amp/lnd150_preamp.png)
of course an isolation transformer is a must
Franco
-
The 'EH5 cathode voltage will be 3.0V-3.5V, not 6V as the schem shows.
(However do NOT try to check this without an isolation transformer!!)
> Amazing to see what was actually sold to the public in years past.
A radio can be built totally enclosed (radio waves in, sound out) so no electric parts are touchable.
A phonograph is possible, though insulation on the pickup pins is doubtful.
A guitar amp, the user MUST touch the circuit, if only at the input plug/jack. The radios et al did not have ZERO leakage. By pushing the leakage spec a guitar amp "could" be made somewhat low hum and not be "legally lethal".
If you study all the "ACDC" g-amp schematics you see a trend of more and more isolation every year. The old rules had been pushed too far and new rules were introduced. It is curious to wonder how far this could have gone. However transistors arrived just in time. They "needed" a PT, but not an OT, so the cost of a fully isolated SS amp fell to the cost of a tube death-trap.
-
50EH5 :w2:
Besides the dangerous nature of these tubes,
2 of them in PP output can put out a blazing 3.8 Watts of power.
You can get that or more from a single little 6AQ5.
I'm finding I am not satisfied with sub 5W output.
PP the little 6AQ5 can give around 10W of output power.
I like that much better. :icon_biggrin:
-
> get that or more from a single little 6AQ5.
At 250V.
Which in 117V lands kinda implies a Power Transformer.
Which is money we can ill-afford on $9.95 radios and $19.95 record players.
Normal people (not musicians) only need around 1 Watt in a sensitive speaker to fill the room.
The first decent cone room-speaker was about 1 Watt amp, but needed 400V to get there.
There was a lot of work done to get the near-Watt with the 95V-115V available from 117VAC through a low-cost bottle rectifier. 25L6 is sorta the king of this low hill, and has ratings to 200V. If you aim lower there are lesser tubes that top out around 1.5W. These mostly need 5V-8V of grid drive. AM detectors and many older needles give a Volt or less. So there's "always" a preamp tube. (Indeed a line of detector/amplifier tubes for radio.)
And then.... the needle makers were not adding cost to raise output. The tube makers were not raising cost to add gain. But the phono-product manager smacked their heads together, saying if $0.50 more needle cost and $0.50 more power-tube cost eliminated $2 of preamp cost, everybody comes out ahead. (Except the preamp tube guy, but he had steady business in radios.) So we have the very hot needles and the very sensitive power tubes. I first saw the 1-stage phono as a stereo with two 60FX5 (SS rect), and it was very sweet.
-
> get that or more from a single little 6AQ5.
At 250V.
Which in 117V lands kinda implies a Power Transformer.
I'm a bit nostalgic for the good old days, myself.
Longing for the old times when men were men, and we all lived a bit dangerously. :l2:
250V does imply a PT, but we are all in agreement that we at least need an isolation PT to use the 25/35/50 series tubes.
So for a little more than a basic isolation transformer, we can get a low voltage PT that will give us a +250V B+.
In todays market, the little 6AQ5 is pretty much as cheap as the other 25/35/50 series tubes.
If we then spend a little more for our PP OT we have a much better guitar amp,
capable of @ 10 Watts of true gritty guitar tube amp power.
I've got a bunch of the 25/35/50 series tubes. If someone is interested in a lifetime supply of them for cheap. :l2:
I haven't messed with them because of their low power output. :icon_biggrin: