Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jkgrooves on November 26, 2024, 09:24:45 am
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I have an amp on my bench that is very unique. It says Silvertone 1346 on the back panel, but it does not look like any 1346 that I could find online. All of the examples that I saw online had two channels, but this one only has one channel. Cosmetically it looks different from others I saw online. It looks more like an amp from the 60s rather than from 1953. Also there are no schematics even for this two channel version. One forum online suggested it is close to the 1396, so I have included that here. There are some similarities, like 4x 6L6, 5U4G rectifier, two output transformers, but there are many differences as well: 1x 12” speaker and 1x 5” speaker, there is no 40uf filter capacitor after the choke, the B+ is the same tap as the screen supplies, R32= 15K here, this has one less tube than the schematic (there is no 12AX7 at the input, instead it goes from the input to a volume pot, then to bass, then a 12AX7 triode, then to a second volume pot labeled Amplification, then to the second triode of the 12AX7, then to the 6SN7 phase inverter) . My current limiter is glowing bright only when the power tubes are installed. I tried another set of power tubes and the same result happened. I tested the resistance on the primary side of the output transformers and both sides of the center tap are equal on one and the other they measure 250ohms and 225ohms. I’ve tested the three filter capacitors and their capacitance matches within 10% of their value and ESR is 1.5-2.8 or so. I plan to change them if I decide to overhaul this amp, but first need to figure out what is shorted out. The amp will need a ton of work... all the coupling and tone caps are waxy and will need to be tested and the previous work done is very sloppy with lots of spliced wires and electrical tape... everything is sticky. But the first order of business I believe to see what is causing the shorting.
V1: 6SJ7GT
V2 & V3: 6L6
V4: 5U4G
V5 & V6: 6L6
V7: 6SN7 (5692)
V8: 12AX7
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Here are some photos:
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No gut shots? It'd be nice to see the inside of the amp where you will be doing the work.
Light bulb test with all tubes out...results? Wattage of bulb in limiter? Tested all tubes?
A schematic isn't really necessary to find a short in an amp like this.
Also, when you start with something that has bad work done inside, generally speaking you have to repair that work first, as your fault could lie there. In my experience, it also pays to clean everything right away. You may find your short during the cleaning process, simply because you're in there working.
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Those tubes position look funny, make sure they are in the right positions, usually the PA tubes are all beside each other.
+1 on the cleaning first !
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Uki it looks like there are 4 mismatched (different) 6L6 tubes - well the pair on the left in pic are the same but the other two look like a mismatched pair of 6L6G tubes.
5U4 inbetween the 4 power tubes.
It's definitely not a Twin 12 1346
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Some friends helped me find this hand drawn schematic for the Danelectro Twin 15, which seems to be the closest match for the amp I have.
https://el34world.com/charts/Schematics/files/danelectro/danelectro_twin15_1961.pdf
If you look at the second page, he mentions the single channel version that I have. Some of the tubes are different (mine had a 6SN7 in place the 6FQ7 and men has a 6SJ7 in place of the 6AUSA), but the main difference is that mine does NOT have a 12AX7 following the input. It has a volume pot following the 68K resistors at the input. My friends suggested that maybe this amp was built for a keyboard of something with a boosted input signal, because the signal will be so weakened by going through the tone stack before any amplification. I have also attached a schematic that I altered to show this.
Anyway, I plan to clean this up and do more testing of the voltages, checking wiring and connections and will report back with my findings. Also, yes the 6L6 tubes that came in the amp are mismatched. I do plan on putting in a matched quad once I get things going. Thanks for the replies so far!