Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: smackoj on December 17, 2012, 02:59:57 am
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I have an old radio chassis from the 50's that I want to convert to guitar amp. The original way the power tranny was wired was to take the Bplus off pin 2 of the rectifier tube and run it through a field coil speaker winding. I could not find what looked like a CT to ground when looking at the orig. wiring. maybe I missed something....highly likely? But now I have what I believe to be two secondary Bplus leads and a third lead, all of which are not connected to anything at this time. When I switch on the power and take readings from each of the 3 leads to ground thru my Fluke meter, I get the following volts AC:
470 vac
160 vac
130 vac
I don't know if I have a bad PT or if one of these leads is supposed to be the CT or what exactly to do at this point? I still know which two leads went to the rect. tube, but those two wires read 470 vac and 130 vac and that obviously doesn't work for a 2-6V6 output amplifier?
Any help or info would be appreciated.
Thanks, Jack D
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Label the leads: 1,2, & 3. Take readings between the leads, rather than ground. Post those readings. If you do in fact have a CT, then you have options as to a full wave rectifier or a bridge rectifier. If you don't have a CT, then you can do a bridge. I'm thinking that you will find one of them to be a CT, and that the radio either had a half wave or full wave rectifier.
Jack
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I am pretty sure you mean resistance readings jack? this amp used a 5Y3 rect. so I think that would make it full wave? here are the resistance numbers. thanks for the help
Note: all numbers are ohms
1 to 2 190
1 to 3 91
2 to 3 99
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Actually, I was meaning for you to put line voltage on the primary, and make voltage comparisons on the secondary leads. You should see fairly quickly if one of them is a CT.
Jack
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OK, let me try this again. All numbers are Vac
1 to 2 760
1 to 3 365
2 to 3 365
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When I switch on the power and take readings from each of the 3 leads to ground thru my Fluke meter, I get the following volts AC:
470 vac
160 vac
130 vac
These readings are meaningless unless you connect one of the leads to ground. For valid readings, measure from wire to wire.
I am pretty sure you mean resistance readings jack? this amp used a 5Y3 rect. so I think that would make it full wave? here are the resistance numbers. thanks for the help
Note: all numbers are ohms
1 to 2 190
1 to 3 91
2 to 3 99
Resistance readings are OK for grouping secondary wires. Since this PT was connected to a 5Y3, there must be a center tap for the HT winding. Your resistance readings indicate that wires 1 and 2 are the HT winding and wire 3 is the center tap.
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I am pretty sure you mean resistance readings jack? this amp used a 5Y3 rect. so I think that would make it full wave? here are the resistance numbers. thanks for the help
Note: all numbers are ohms
1 to 2 190
1 to 3 91
2 to 3 99
3 is CT.
field coil shaves off about 100V give or take.
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OK, let me try this again. All numbers are Vac
1 to 2 760
1 to 3 365
2 to 3 365
++ What DL said. If these readings are unloaded, there will be a drop when under a load. I'll guess 340->350vAC, with a load. Maybe less. What other tubes are in the radio?
Jack
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all the tubes are octal in this amp. It had some kind of a strange fixed bias that I couldn't figure out with my amt of knowledge, but it was definitely fixed (no common R or cathode Cap). It used a 5Y3 then down the line one 6V6, then a 6SQ7, then the 2nd 6V6. The rest of amp/radio is filled out with a couple 6J5s, 6SK7s, another 6SQ7 and a weird one or two. Everything metal cased except the V6s and the 5Y3 which are glass.
This amp was labeled "Firestone" like the tires but I'm gonna guess they bought it from another mfg. and put their label on it?
Message rec'd on wire no. 3 being the CT......got that corrected and working as it should.
thx, JD :icon_biggrin:
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> strange fixed bias
The PT HV CT goes to the - end of the first filter cap, then to a ~~300 ohm resistor, *then* to common ground.
The current from all the tubes causes the CT end of this resistor to run negative of ground. 6V6 grid is tied to this negative voltage. The grid of the 1st Audio and any Magic Eye might return to taps on this resistor.
Taking 6V6 as 40mA and RF/IF demand as 15mA-5mA, the bias is 3/4 about the 6V6's demand. Pulling the RF/IF demand through the same resistor makes it "25% fixed" bias. This is mostly a hangover from older smaller power tubes.
2K 10W would be a fair replacement for the field coil.
A field-coil speaker and 6V6 strongly suggests 1938-1942, not "50s". There was a 1947 Firestone with FC speaker, probably using-up old parts.
If you are keeping the original OT (it may be very fine), keep the B+ down toward 300V. Over-voltage will force under-biasing and poor load match. 250V-300V ought to be good.
It is possible the output bottle should be 6F6. 6V6 would be a slap-dash replacement. It is an "improved 6F6", but needs re-bias for best performance. 6V6 is fine/better now. Since you have lost the cathode(s)-bias resistor, just wire and bias it like any 300V Champ.
It might have enough ooph to pull two 6V6, but you would need a new OT.
Firestone-branded radios ran about 1938 through 1963. As you say, they were probably badge-jobs. There were many-many small radio makers and many more cabinet-makers who would compete for such work.
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thanks PRR. the bias scheme makes sense now. i like the fact that there are quite a few octal sockets in very good condition which opens lots of doors for possible circuits. i have done some tweaks with the layout and put in a simple, Champ-like circuit using one 6v6, one 6SN7 and a 5Y3. I am reading 311 volts dc on the pow tube plate at this time. i will do some more resistor tests in the power section today. right now both plates on the SN7 are very low at around 50 vdc so that needs a boost too.
gracias amigos, JD