RG50TC combo very low output..

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nsbluesman
RM20
Posts: 3
Joined: Sun Nov 28, 2010 6:14 am

RG50TC combo very low output..

Post by nsbluesman » Sun Nov 28, 2010 6:29 am

Hi All:

I have a RG50TC opened up on my bench right now and it is puzzling me, all tubes have been testted and swapped out for known good ones, heater voltages check out, VDC on tubes 1, 2 VAC on 3,4,5,6. Plate voltages appear normal for a 6L6 AB amp. The gain channel has volume, not sure if I would say normal, but definitely way louder than the clean channel. with the volume all the way up on the clean channel it's really really quiet. I have a copy of the schematic, but there aren't too many voltage refs listed, complicated little sucker and I would love to be able to try it out at full power.

Any thoughts or suggestions of where to start looking, I did see some posts about the bridge rectifier in the heater circuit for valves 1 and 2, but the voltages look good..

Puzzled in Nova Scotia..

Metallifornia
RM20
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Joined: Sun Nov 28, 2010 2:26 pm
Location: Harrisburg PA

Post by Metallifornia » Sun Nov 28, 2010 2:42 pm

Start by plugging into both of the inputs to see if there is signal passing both inputs. The difference between high and low inputs is 68Kohm (low) or 32Kohm (high) input impedance. I have had amps across my bench in the past where guys have burned up the input resistors by trying to plug in other amps or really hot signals from other effects. If they read ok then go to relay "RL1" it switches between the overdrive and clean channels by "shorting" the signal of the "off" channel thru R1 (10Ohm) to ground. Be sure that you have continuity (and signal passing thru) on the the appropriate pins when you switch channels. Try the Boost switch. There is a FET labeled Q1 with a 1Mohm resistor in parrallel in there when activated it creates a voltage divider thru R207. Make sure the FET is functioning.

My bet is the input resistors or the relay. Just because I have to ask, where is the master set during your tests?

-Brian

p.s. Here is a list of Randall's that I own. Big fan!
Randal RT30C, RH50T (6l6GC MOD), RG50TC, RGT100 and RG100SC
If it ain't broke, let me take it apart and find out why it isn't

Metallifornia
RM20
Posts: 3
Joined: Sun Nov 28, 2010 2:26 pm
Location: Harrisburg PA

Post by Metallifornia » Sun Nov 28, 2010 3:02 pm

I just noticed you said 6L6 in your RG50TC. Did you modify the bias circuit to give a wider voltage sweep on the grids?? That could be the source of the low volume. The great thing about these amps is that they have external bias pots, testpoints and 1 Ohm cathode resistors. Just be sure to use the simple power formula when you bias so that you don't push the output transformer too hard. Dissipated Power=Plate voltage x Cathode Current.

just another thought

-B
If it ain't broke, let me take it apart and find out why it isn't

nsbluesman
RM20
Posts: 3
Joined: Sun Nov 28, 2010 6:14 am

Post by nsbluesman » Sun Nov 28, 2010 7:34 pm

Interestingly enough, when I go from the loop out to a loop in on another amp, tons of volume, so I definitely have issues in the output stage.. I am going to pop some good EL34's back in and rebias again. The master was set full on, I have chased down the 15v rails, checked heaters, checked plates etc.. very strange. I will check the relay you are pointing out. Thanks

Colin

Metallifornia
RM20
Posts: 3
Joined: Sun Nov 28, 2010 2:26 pm
Location: Harrisburg PA

Post by Metallifornia » Sun Nov 28, 2010 7:56 pm

Go for the Effects loop level switch on the back (sw4) it switches in 1/2 of IC3 (a 4558 op-amp). The first 1/2 is a buffer which is always running and the second half is an voltage amplifier which gets switched in to help compensate for long cable runs. You could swap IC3 with IC2 the reverb driver op-amp they are the same 4558 op-amp. My RH50 head started to sound like the gain would trail off after a couple seconds of sustain on a chord. The switch contacts were going bad. I still occasionally have to give it a jiggle to get it to full operation.

For the 6l6 setup (which I think sounds better) just short R147 and adjust bias to your taste within 50W limit of the output transformer. I have been playing mine with 6l6's for 2 years with no problems.

Are you running a 12AT7 for the phase inverter?

-Brian
If it ain't broke, let me take it apart and find out why it isn't

nsbluesman
RM20
Posts: 3
Joined: Sun Nov 28, 2010 6:14 am

Post by nsbluesman » Mon Nov 29, 2010 6:29 am

Thanks Brian,

I will check that when I get home. Yep 12AT7 in PI spot, I am just hoping I don't have an issue with the Output tranny. At least I am down to trouble shooting the output stage past the loop now. I have built a 5F6, 5E3, mooded Traynor YBA-1's fixed lots of other amps, but when I looked at this schematic I thought uh-oh....

Cheers

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