by Rob-NYC »
Thu Jul 16, 2015 4:47 am
Some general thoughts on your observations and pic's.
-The speaker has been changed. the original was a field coil type. The replacement is permanent magnet. When this is done it removed a large load from the B+ power supply and the voltages go up considerably. The standard solution is to place a resistor as a dummy load across the power supply as a sub. looking at the underside of the amp, I see a large green resistor right near the 5U4 socket. This is not original and is probably the dummy load. The resistance should be approx 6k-ohm.
There is no way to tell what the type or efficiency of that replacement speaker is by the pic. Since 15" is not a commonly used size outside of musical instrument amps, it is possible that is what is in there. If so it will be efficient, but very lacking is anything above 2-3kHz. In any case, a simple addition of a crossover and tweeter is necessary to make full use of the new cart and for pleasant listening. Our perception of "good sound" has vastly improved in the intervening 60 years since this machine was built.
If you go with an original speaker, it will most likely have to be reconed and the amp mods, which may be very simple, be removed.
-Low volume. Starting with the new cart used here. The 345-03d is far superior to the original black or red head. however it's output is considerably lower. unless the preamp is modified for greater gain, maximum level will be a -bit- lower. It should not require a full-up volume control.
Weak tubes or low B+ along with an inefficient speaker can also cause low volume. I note that the bass and treble controls are not full on and this will reduce apparent volume.
It is not necessary to hire someone to test tubes. That is a waste of money and time. The tubes that directly affect gain are the 6J7, 6SN7 & 6SL7. A better use of funds is to simply buy new ones, both for test and spares. A 5U4 is a good idea too.
Given that the amp appears to have been rebuilt there is no good reason to go over it again. A few voltage readings will tell you if there are any weak areas.
If you wish to use a wallbox, the stepper will have to be serviced. It may have been done when the units were restored, but the rectangular 5Mfd cap on the right side of the unit must be replaced. NBD.
There are some other issues to consider, such as whether the tonearm has been rebalanced to accommodate the lighter cart but over-all I'd say that given this is one of the most desirable 1950's Seeburgs and has been tastefully restored (no Betty Boop or Elvis graphics) at 2K or less you have a good deal.
Rob
"If we believe absurdities, we shall commit atrocities" -- Voltaire