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RCA MI-9377 amps are here.
- Thread starter Ken Boyd
- Start date
Lavane
Lunatic Member
Triode strapping a pentode cuts the output of the tube by half.
I have an amp and a couple PPP mono blocks with switches that allow you to switch between ultra linear and triode mode.
Personally I never flip those switched to triode mode. It just cuts the output.
Some people really like triode strapped tubes though. I just finished a single ended EL34 amp thats triode strapped.
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Feasible sure, sensible maybe?
At the 100 ohm resistor feeding the screen, unhook it from the power supply line coming from the two gas regulators and connect it to the plate at each tube. Probably have to alter the cathode bias resistor since the screen voltage will be higher tied to the plate, but thats really about it.
As-drawn the screen supply is about 260 volts. Not sure what the plate supply is, but it looks like a 350-0-350 transformer, with a pair of 5U4G's I'd guess its going to be close to 380 vdc.
Across the tube once the cathode voltage is pulled out, it will end up about 360 on the plates and 250 or so on the screens, so slightly shy of typical datasheet values I expect.
At the 100 ohm resistor feeding the screen, unhook it from the power supply line coming from the two gas regulators and connect it to the plate at each tube. Probably have to alter the cathode bias resistor since the screen voltage will be higher tied to the plate, but thats really about it.
As-drawn the screen supply is about 260 volts. Not sure what the plate supply is, but it looks like a 350-0-350 transformer, with a pair of 5U4G's I'd guess its going to be close to 380 vdc.
Across the tube once the cathode voltage is pulled out, it will end up about 360 on the plates and 250 or so on the screens, so slightly shy of typical datasheet values I expect.
That seems like adding a switch to select between the two modes would be easy nenough expect for having to have the switch change the bias resister. Maybe use a two pole switch one to drop the 100 ohm resister and connect to the plates, and the other pole changing resister values.
On the power supply line and switch to the plates, I wonder if I would need to use a relay to control the switching so that the switch isn't dealing with any high voltage.
On the power supply line and switch to the plates, I wonder if I would need to use a relay to control the switching so that the switch isn't dealing with any high voltage.
I would not advise switching between triode and pentode with the amp on, that would seem like an invitation for very bad things to happen.
oh absolutely, also most switches and relays are not rated to break high voltage DC. I'd also expect a lot of unhappy noises.
you could possibly get around the bias shift thing by applying some negative bias to the grids. Looks like there is an un-used 6v winding. With a voltage doubler it may produce enough to be able to correct for the extra current draw. Could switch the grids between 0v and some negative voltage and leave the cathode resistors alone.
you could possibly get around the bias shift thing by applying some negative bias to the grids. Looks like there is an un-used 6v winding. With a voltage doubler it may produce enough to be able to correct for the extra current draw. Could switch the grids between 0v and some negative voltage and leave the cathode resistors alone.
The paint department head got my stencils done so I can paint my selector switch markings on the amp.
Unfortunately the water jet guy didn't get my bottom plates cut for me, so I could get the plate and feet installed. He said he would get to it next week, but I have to go out of town for a few days to do a site survey for a job. Will get it by the end of the week.
Unfortunately the water jet guy didn't get my bottom plates cut for me, so I could get the plate and feet installed. He said he would get to it next week, but I have to go out of town for a few days to do a site survey for a job. Will get it by the end of the week.
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I am concerned that maybe the service wiring was not correct on this from when I received it. Yes it played music but I was concerned about the way the 120 volt primary service wire is wired. There is a terminal strip inside and they had the black lead of the plug wire landing on the H terminal and the Netural wire landing on the L terminal. On the back side of that the H terminal goes to the fuse holder and then there are three wires coming out of the main power transformer, a red and black, yel and black, green and black. It is connected to the red and black lead of the primary, but my drawing shows from the fuse to land to the yellow and black. I am thinking, well kind of guessing, that those three wires are for picking what voltage you want to use. Maybe the green and black is for 100 volt, the yellow and black would be for 120, and maybe the red and black is for 230 volt, but I really am not sure of that. Anyone want to help guess? Maybe a more experienced guess.
Maybe I should just check output voltage and see if it is the right voltage ration for the input voltage that I apply. I have it on a variable voltage source right now.
That blue wire is coming from the switch, and I have rewired it to be coming off the H terminal and connected to the neutral wire. And I connected the postive lead to the L terminal which then goes to the black wire and goes directly into the primary transformer.
Maybe I should just check output voltage and see if it is the right voltage ration for the input voltage that I apply. I have it on a variable voltage source right now.
That blue wire is coming from the switch, and I have rewired it to be coming off the H terminal and connected to the neutral wire. And I connected the postive lead to the L terminal which then goes to the black wire and goes directly into the primary transformer.
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I also am under the thought that the L terminal should be the load and not the neutral, and the H terminal should be the neutral. So should the neutral lead be for the fuse protection on the primary transformer?
I did a primary resistance check between the black wire and the three wires on the primary input side, Green/black=1.2 ohms, yellow/black=1.4 ohms, and Red/black=1.6ohms.
I did a primary resistance check between the black wire and the three wires on the primary input side, Green/black=1.2 ohms, yellow/black=1.4 ohms, and Red/black=1.6ohms.
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Normally the fuse is on the hot lead, and neutral just runs straight to the transformer.
Since all of those wires read to one another its a single primary. Easy way to see if its sensible is to apply voltage and see what the heater winding comes out as. Run it up on a variac until you get 6.3 vac and see what that ends up being.
Since all of those wires read to one another its a single primary. Easy way to see if its sensible is to apply voltage and see what the heater winding comes out as. Run it up on a variac until you get 6.3 vac and see what that ends up being.
Thanks Gadget, but I think I figured it out, just due to my thinking out loud, she is sing perfect now, zero hum, I needed to change my polarity on the audio input. That eliminated the hum, I had gone by the wiring that was still attached to the amp which had TB1 as the negative or ground of the input and terminal 3 as the hot going into the unit. But looking at the drawing, I saw that the input hot which goes to the grid is terminal one so just reversing the polarity and all hum was gone, and now I was able to land the ground on the service lead so the chassis is now grounded. The drawing doesn't show polarity on the inputs or the service leads.
I went with the Yellow and black lead to the fuse lead as that is what the drawing shows, and at 115 volts my output on the secondary red wires to the 5U4Gs is right at 350 volts.
I went with the Yellow and black lead to the fuse lead as that is what the drawing shows, and at 115 volts my output on the secondary red wires to the 5U4Gs is right at 350 volts.
Checked all the voltages seems all good, right at 350 at the pins 4-6 on the rectifiers, 6.2 at the heaters on the 6L6 g tubes and using the selector all the tubes fall in the green zone. I did install a CL-90 Amphenol Inrush limiter on the power supply.
Swapped out the tubes on the 6L6g's and the 5U4's with some old NOS tubes and everything seems to be behaving itself and sounding very good.
Still have to install the IEC connector, 5 way speaker binders, and RCA on the back side.
I was out of town on a job so hopefully the water jet guy cut bottom covers for the two amps so I can install them and some feet.
My paint job on the selector numbers didn't come out perfect but it kind of matches the weathered look of the old amp.
Swapped out the tubes on the 6L6g's and the 5U4's with some old NOS tubes and everything seems to be behaving itself and sounding very good.
Still have to install the IEC connector, 5 way speaker binders, and RCA on the back side.
I was out of town on a job so hopefully the water jet guy cut bottom covers for the two amps so I can install them and some feet.
My paint job on the selector numbers didn't come out perfect but it kind of matches the weathered look of the old amp.
Relay? Do you mean the input transformer? It certainly could be limiting bandwidth, greatly depends on the quality of the transformer. Based on the drawing its probably running as a step-up, not sure on the ratio but you may need a decent bit more input signal to get full output if its bypassed. Doesn't look like a whole lot of open loop gain here.
depends which 1, 2, 3 we're talking about
On the terminal strip, it wouldn't be for balanced. 3 is ground, 1 and 2 are tied to the same point on the transformer, just with a 130 ohm resistor in there for whatever reason. Not sure what that would do, it wouldn't drop the signal enough to make it useful as a high level / low level input. Maybe its to pass the signal on to the next thing down the line but provide a little isolation?
If you mean on the transformer itself, ground #2, and input to 1 and 3 would work for balanced input. Assuming 2 is indeed a center tap. If its not, and its 500 ohm from 1 to 2, its probably for booster amp duty.
or maybe all of these things are true depending on what is needed for a given install. Its far from impossible they would have set this up to allow for multiple connection options just by moving a few wires around.
On the terminal strip, it wouldn't be for balanced. 3 is ground, 1 and 2 are tied to the same point on the transformer, just with a 130 ohm resistor in there for whatever reason. Not sure what that would do, it wouldn't drop the signal enough to make it useful as a high level / low level input. Maybe its to pass the signal on to the next thing down the line but provide a little isolation?
If you mean on the transformer itself, ground #2, and input to 1 and 3 would work for balanced input. Assuming 2 is indeed a center tap. If its not, and its 500 ohm from 1 to 2, its probably for booster amp duty.
or maybe all of these things are true depending on what is needed for a given install. Its far from impossible they would have set this up to allow for multiple connection options just by moving a few wires around.
After just about finishing this, I wish I would have orientated it so the side was the front, so the widest length was the depth, puts the big transformer in the rear, and the 6SL7, the gas tubes in front and the 6L6G's running down the right side, takes less room on the shelves.