Randall RG200 Valve dynamic burning heater resistors

[reflection77ca2000@yahoo.ca]

Randall

It is not giving DC voltage. Maybe that is the problem! Reads 12AC. Randall says 12.6ACV but if I should be DC, that would explain right?

[Robert M. Martinelli]

That's why I highlighted the oddity of the two 100 Ohm resistors apparently used in a DC heaters' supply arrangement - If it's AC, the resistors have all the right to be there, providing an "artificial" CT to GND. If it's DC, you can throw them away, as they're useless.

It indeed seems the schematic has errors in it

Cheers

Bob

[52 Bill]

I've looked at the schematics and I don't see the tube dc heater supply anywhere here.

Maybe someone could tell me where it's shown here.

Is there another drawing that shows the transformer secondaries?

[Gtr_tech]

My bad. I was misreading the cap voltage at the top of the pwr supply schemo. I read them as 6.3v not 63v. So thats the supply rails for teh output section. There must be another page that details the xfmr connections. The heater on the tube is ran from 12v 'cause pin 9 is NC. The 1w rating on those 100r resistors leads me to believe they are there to drop some voltage, not for gnd reference.

Got the other page of the schematic? That'll tell the rest of the story. <insert Paul Harvey joke here>

[Robert M. Martinelli]

.....The 1w rating on those 100r resistors leads me to believe they are there to drop some voltage, not for gnd reference.

Got the other page of the schematic? That'll tell the rest of the story. <insert Paul Harvey joke here>

Hi Gtr_tech!
If that was their intended purpose, then only one series ( dropper ) resistor would have been enough to accomplish the same goal. They indeed look like a virtual CT network. If the HTR supply is 12.6VAC, then 6.15 VAC are dropping across each resistor to ground, so 6.15V/100Ohm=61.5 mAmps, multiplying this current by 6.15V yields some 0.4W, and, since this power is close to the max power a 1/2 W resistor can safely dissipate, they used 1 W ones to play it safe IMHO.

I still believe the schematic has errors in it, because of the HTR supply carrying the + and - signs, and the presence of an artificial CT network - the two things should be mutually exclusive IMHO.

Cheers

Bob

[Robert M. Martinelli]

...The heater on the tube is ran from 12v 'cause pin 9 is NC. The 1w rating on those 100r resistors leads me to believe they are there to drop some voltage, not for gnd reference.

Got the other page of the schematic? That'll tell the rest of the story. <insert Paul Harvey joke here>

Hi Gtr_tech!
No disrespect intended, but, if that was their intended purpose, then only one series ( dropper ) resistor would have been enough to accomplish the same goal. They indeed look like a virtual CT network. If the HTR supply is 12.6VAC, then 6.15 VAC are dropping across each resistor to ground, so 6.15V/100Ohm=61.5 mAmps, multiplying this current by 6.15V yields some 0.4W, and, since this power is close to the max power a 1/2 W resistor can safely dissipate, they used 1 W ones to play it safe IMHO.

I still believe the schematic has errors in it, because of the HTR supply carrying the + and - signs, and the presence of an artificial CT network - the two things should be mutually exclusive IMHO.

Cheers

Bob

[reflection77ca2000@yahoo.ca]

Randall

Somebody out there must see what is happening. Heater circuit is definitely AC, comes right off transformer.
CN2 goes to preamp. I thought maybe some strange voltage was coming in so I disconnected it. I ran I ground and resistors began to burn. Getting better at this. Instead of killing resistors first try, I am able to test about ten times before they are toast.
Amplifiers aside, what Is actually happening here. Too much current?
Short to ground? It looks like resistors have a direct route to ground.
What am I not seeing? What in this circuit would alter the current through resistors besides heater voltage?

[Gtr_tech]

What happens with the tube removed?

[reflection77ca2000@yahoo.ca]

Randall

Resistors still burning. Heater is definitely AC. Heater wires coming directly from transformer. Noticed something interesting. Heater wires to Ground. One is 12vac the other is 9vac

[reflection77ca2000@yahoo.ca]

When heater wires to ground. One is 12vac the other is 9vac

[reflection77ca2000@yahoo.ca]

RG200

Schematics are corect just misleading eg: QC1 on power supply is irrelevant to QC1 on power amp.
Nevertheless. I found and inconsistency between heatr leads. They come directly from trans. I disconnected transformer from everything.
Assuming a pair of blues, a pair of greens and a pair of reds should be the same, then I have serious transformer problems. Because the blue are 51vac each, the green are 93vac and 136vac, the red: heater wires are presently 20vac and 31vac. All using a ground wire from transformer.
2 green
2 blue
2 red
1 black

[reflection77ca2000@yahoo.ca]

Randall heater resistors burning

Hi again and thank you for replies. Schematic is definitely out there and unclear. I am at the transformer now which is kind of chaotic in respect to output. Randall says it had transformer problems but knows very little about it they said there are underrated caps in transformers I can't see any plain to the eye inless they are in the core.
Any suggestions on how to confirm transformer preoblem or anybody have RG200 Transformer nearby that can give me voltages?

[reflection77ca2000@yahoo.ca]

RG200

While looking for defective components in transformer (which I found nothing) I noticed a few of the secondarys touching internally. I separated them and turned on transformer. While voltages are not what I expected, they had definitely changed. I have reassembled amp. Resistors are not burning. I have yet to connect speakers and listen. Any tips or feedback before I continue. Randall says they have had numerous transformer problems this may be the common issue. Secondary soldid copper is soldered to leads and not secured well, just taped into place side by side.
If this works I wil epoxy leads into place.

[reflection77ca2000@yahoo.ca]

Darn. Prior to this stage, when it was still burning resistors, the amp would burp out lound intermittent guitar. Now it is functioning flawless sounding great but only about 2 watts of power. Outputs or tube?

[reflection77ca2000@yahoo.ca]

12at7wc

Just finished repairing amp. Transformer was mess up and burned heater artificial ground resistors. Now amp has very little poer although sounds good. Wondering if tube suffered damage. Heater is 18Ohms. I would generally except a filament to burn open but I have little tube experience.