Hi,
New poster here looking for some advice on this Fender Studio Bass amp I'm restoring. If you're unfamiliar with the amp it's basically a Super Twin slightly reconfigured for bass. I'm having a problem with the heater filament system and have exhausted my knowledge, so I hope someone familiar with these amps could help.
My issue is that the heater filaments going to the 6 x 6l6 sockets have all the voltage on one side. IE on a given socket one heater pin will have the whole AC voltage of about 6.8vac and the other pin has 0vac.
I first thought the issue was in the hum balance circuit (which on late 70's Fender amps is separate from the bias balance, and is used to balance heater voltages). The pot was bad, but after fixing that circuit up it seems the balance only affects the pair of white wires from the power transformer which heat the preamp tubes I believe. Operating the pot I can match the voltages on those taps to like 3.6vac.
But the 6l6 heater filaments go to a separate terminal strip. One lead is connected to the green/yellow wire from the power transformer, and the other is connected to the green wire. But what's weird is that the terminal with the green tap is connected directly to ground, so no AC voltage flows to the heater filament and thus those pins. I pulled the green and green/yellow leads and measured their voltage and both are giving off 6.8vac, so it's not the transformer. Did the factory just miswire the amp back in the day? Should both heater filaments be connected to the green/yellow rather than one to green and ground?
Attached is the schematic, which shows the green tap going directly to ground. Thanks for any help/clarification you can provide!
Nate
New poster here looking for some advice on this Fender Studio Bass amp I'm restoring. If you're unfamiliar with the amp it's basically a Super Twin slightly reconfigured for bass. I'm having a problem with the heater filament system and have exhausted my knowledge, so I hope someone familiar with these amps could help.
My issue is that the heater filaments going to the 6 x 6l6 sockets have all the voltage on one side. IE on a given socket one heater pin will have the whole AC voltage of about 6.8vac and the other pin has 0vac.
I first thought the issue was in the hum balance circuit (which on late 70's Fender amps is separate from the bias balance, and is used to balance heater voltages). The pot was bad, but after fixing that circuit up it seems the balance only affects the pair of white wires from the power transformer which heat the preamp tubes I believe. Operating the pot I can match the voltages on those taps to like 3.6vac.
But the 6l6 heater filaments go to a separate terminal strip. One lead is connected to the green/yellow wire from the power transformer, and the other is connected to the green wire. But what's weird is that the terminal with the green tap is connected directly to ground, so no AC voltage flows to the heater filament and thus those pins. I pulled the green and green/yellow leads and measured their voltage and both are giving off 6.8vac, so it's not the transformer. Did the factory just miswire the amp back in the day? Should both heater filaments be connected to the green/yellow rather than one to green and ground?
Attached is the schematic, which shows the green tap going directly to ground. Thanks for any help/clarification you can provide!
Nate
I don't really get what the question is. As shown on the schematic, the filament supply for the preamp tubes (white leads) are balanced. Twisting and balancing them will cancel most of the EM field which causes interference in sensitive parts of the amp. This is most important in the preamp section.
The supply for the 6L6's however IS referenced to ground. So 6.3 or thereabouts AC RMS volts on one lead and ground on the other. Why do you think it wouldn't work like this? There's the same amount of energy available to heat up the filaments. Just two ways of achieving the same thing, although the one above does have its benefits as explained.
The supply for the 6L6's however IS referenced to ground. So 6.3 or thereabouts AC RMS volts on one lead and ground on the other. Why do you think it wouldn't work like this? There's the same amount of energy available to heat up the filaments. Just two ways of achieving the same thing, although the one above does have its benefits as explained.
My understanding was that the heaters pins 2 and 7 of the power tube sockets should each have AC voltage on them. So it is normal to have 6.8vac on say pin 2 and 0vac on pin 7?
The reason I'm asking is because the amp seems to be outputting less than its rated power, as measured in a general way with a 1khz and 400hz sine wave in, and measuring voltage across the speaker out. If it is normal to have 0v on one heater pin maybe I'll move on to checking the coupling caps in the power section. Thanks for your help.
The reason I'm asking is because the amp seems to be outputting less than its rated power, as measured in a general way with a 1khz and 400hz sine wave in, and measuring voltage across the speaker out. If it is normal to have 0v on one heater pin maybe I'll move on to checking the coupling caps in the power section. Thanks for your help.
Not quite. it is normal for pins 2 and 7 to have 6v BETWEEN them. What their reference to ground is doesn;t matter, at least not as far as heating the tube. VARIOUS grounding schemes are used in amps for hum reduction. Fender had the balance pot on some, a real center tap on some, and a artificial center tap on many. Other tricks are to reference the heaters top some positive voltage instead of ground, and in cathode biased power stages, they sometimes used the cathode voltage for that. Hey, it is just sitting there waiting to be used.
Heater hum mainly affects preamp tubes, so most of these approaches are not needed for power tubes.
Your schematic clearly shows a separate 6v winding for the 6L6s with one side grounded. So your amp is wired exactly as the schematic requires.
Heater hum mainly affects preamp tubes, so most of these approaches are not needed for power tubes.
Your schematic clearly shows a separate 6v winding for the 6L6s with one side grounded. So your amp is wired exactly as the schematic requires.
Which output transformer does yours have? There were 4 and 8 ohm versions (see threads on Talkbass regarding this), and two different output power specs that I know of. The 8 ohm version that I worked on made ~160 clean watts with then-current JJ 6l6s, FWIW.
It's the 8ohm transformer one. The back panel claims 200w, but that might be an optimistic view. If you're curious I can update with new measurements when I finish tinkering with the power section.
On my schematics the SuperTwin titles are "180-watt Super Twin Reverb" or "180-watt Super Twin" and on the Studio Bass my revision A schematic title is "180W Studio Bass" but your revision D schematic is "200 watt Studio Bass w/ LED".
180W Studio Bass Rev.A
Output transformer 012413 8 ohm 180w spec repeated at trans
Power transformer 014679
200 watt Bass Amplifier Rev.D w/ LED has the later Fender/Rogers/Rhodes logo
Output transformer 012991 8 ohm 200w spec repeated at trans
Power transformer 012405
Power supply main B+ circuit is identical and identical 500v. into identical 6 6L6GC so indeed the difference seems to be the output transformer.
Both have the ultralinear tap from the output transformer.
So does yours have a switching TS 1/4 foot jack for a 1-button footswitch or a TRS 1/4 foot jack for a 2-button footswitch?
When I got my Super Twin one tube had glowing corners on its plate...just FYI.
180W Studio Bass Rev.A
Output transformer 012413 8 ohm 180w spec repeated at trans
Power transformer 014679
200 watt Bass Amplifier Rev.D w/ LED has the later Fender/Rogers/Rhodes logo
Output transformer 012991 8 ohm 200w spec repeated at trans
Power transformer 012405
Power supply main B+ circuit is identical and identical 500v. into identical 6 6L6GC so indeed the difference seems to be the output transformer.
Both have the ultralinear tap from the output transformer.
So does yours have a switching TS 1/4 foot jack for a 1-button footswitch or a TRS 1/4 foot jack for a 2-button footswitch?
When I got my Super Twin one tube had glowing corners on its plate...just FYI.
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