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Old 19th January 2009, 03:47 PM   #101
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That's how I'd do it.

The resistor above the CCS seems like it may not be necessary.

Your mosfet output is going to swing from -41V to 0V, so +/- supplies for the mosfet need to exceed those numbers.

It seems like Eli has used IRFBC20 in similar situations, ask him how much voltage it needs to perform well. I use FQPF2N80, which is another option. Isolated case and Crss is quite low for a fet that can handle that much power. I always give it plenty of voltage, so I don't know how it performs if you try to squeeze it down.
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Old 20th January 2009, 12:11 AM   #102
chrish is offline chrish  Australia
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Thanks SpreadSpectrum,

The front end is a direct copy of Eli's "El Cheapo", so can't comment on the resistor above the CCS, in the original it is a 10K 1W CF. My knowledge of solid state devices is even poorer than my knowledge of vacuum tubes, so this is a bit of a challenge for me If this topology is looking OK, I have to work out how to power it. I think I am going to go the way of regulated B+. That will require then a regulated -ive bias. The power transformer I have does have a 50 volt tap, so that should be OK for the bias supply (?). I need a little guidance for the power supply of the mosfet. Do I need a totally separate supply for them? Can I feed the -ive supply for the mosfet from the -20.5 volt output tube bias? Can the +ive supply come from a tap from the regulated B+? What grid current should be designed for?

I am sorry for the amount of questions and guidance being asked for, but I think I have a head like a sieve at the moment!

Cheers,

Chris
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Old 20th January 2009, 12:30 AM   #103
SY is offline SY  United States
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Quote:
The resistor above the CCS seems like it may not be necessary.
I don't know about Eli, but I use the resistor there for two reasons:

1. To take up some of the dissipation from the CCS.

2. To somewhat isolate the CCS's capacitance from Cdg.
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Old 20th January 2009, 02:43 AM   #104
tikiroo is offline tikiroo  Australia
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The resistor above the CCS in the original El Cheapo was 10k as it had to drop a lot of voltage to limit dissipation in the CCS (the negative supply was > -100V if I recall correctly). I am successfully using a -60V supply with a 1k resistor in this position.
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Old 20th January 2009, 03:10 AM   #105
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Both SY and Tikiroo have made cogent remarks. I specifically call for a Carbon film part between CCS and the cathodes to provide some thermal isolation for the "sand". Why take a chance on conducted heat damaging the FETs?
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Old 20th January 2009, 03:16 AM   #106
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Quote:
I need a little guidance for the power supply of the mosfet.
OK, there are three places in your circuit that need a negative voltage source. It is possible to derive them all from the same negative supply but I doubt that the bias tap on the power transformer will work.

First off the CCS in your schematic is shown as being connected to ground. Depending on the choice of input tube and bias points the CCS will only have a volt or two to work. Morgan Jones made this work using an LM124 and a 6J6 tube, but most CCS's need more voltage. My old favorite the IXYS 10M45 needs at least 15 volts to be happy redardless of what the data shees says. All semiconductors exhibit voltage variable capacitance effects which are reduced by raising the voltage. You need something between -15 and -450 volts if using a 10M45. Regulation is not important.

The mosfet source is connected through a resistor to a negative voltage source. This voltage must be more negative than the peak negative drive voltage. In reality you need quite a bit more voltage. You need enough voltage to assure cutoff of the output tube. I like to have enough voltage to assure continuous current through the mosfet as the tube approaches cuttoff. The mosfet can source plenty of current into the grid of the output tube for AB2 or instantaneous overload, however the source resistor and negative voltage source must discharge the miller capacitance of the output tube to recover from overload. This is not such a big deal on a pentode , but is a big problem on say an 811A. I would go for - 100 volts or more. Regulation is not important, but the supply should be clean.

The bias pot is connected to a negative voltage source. Again I like to be able to adjust the output tubes to cutoff when bringing up a new amp, so you need at least -45 volts here. It was stated previously that the bias supply must be regulated if the plate supply is regulated. In reality the plate supply is not important it is the screen supply that controls the current through a pentode. The screen and the plate are the same supply in UL or triode, so if this supply is regulated, the bias supply must be regulated. The entire supply does not need to be regulated, just the source for the bias pot. A zener or gas tube is sufficient.

Why can't you use the bias tap on the power transformer? The Hammond transformers only have the tap on one side of the center tap (one purple wire). This means that you would only get to use a half wave rectifier. Normally this is used to supply a few microamps to a bias pot, so a half wave rectifier is common. However the mosfet works best if fed a few milliamps, which may make getting a clean supply difficult from a half wave rectifier.

What would I recommend? Well I have built several PowerDrive test boards that have been used in 6L6 type amplifiers among others. You are also going to need a positive voltage source to feed the drain of the mosfet. The easiest way to get there is to use a small isolation transformer that has two 120 volt windings. Wire them in series to make a 120-0-120 volt winding. I use a bridge rectifier with the center tap grounded to make +150 and -150 volts in a manner similar to most solid state power amps. This feeds the mosfet its + and - supplies. It also feeds the negative voltage to the CCS on the input stage. I use a zener diode and a resistor to make -68 volts from the -150 volt source.

I like the LTP type of phase splitter. I tested it using several types of tubes when developing the Simple P-P. I found that the gain was barely adequate to drive a pair of EL84's. There was not enough gain to allow the use of negative feedback or triode connection. I have not tried the 7591 tube, but I am not sure that a single stage LTP will be enough if you ever want to try 6L6's or other tube types, and it won't be enough if you want to run 6L6's in triode, which is what I found to be my favorite choice to get about 20 watts.

Quote:
Your universal driver looks VERY interesting. This project is going to take a little time, so will follow your schematic development with great interest! Since my space in the chassis is not that great, I think that waiting for the development of the driver PC board would work well for me.
I have been working on this driver on and off for a long time. There are several influences in my life right now that have killed much of my tube time lately. I have realized that I went through all of 2008 without completing a single amplifier. That won't happen in 2009. I can't say when the PC board will be completed, or if funds will be available to get a batch made, so don't wait on me. I have just finished laying out the first proto board. I made two of them and have started populating one of them.

I have already received a few emails asking questions. Its late here and I need to get up early tomorrow, but I will start another thread at my next opportunity to post progress, testing, photos, and yes, schematics.
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Old 20th January 2009, 06:09 AM   #107
chrish is offline chrish  Australia
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Thanks for taking the time to post such a comprehensive response George. Much appreciated. Yes, I made a mistake in my representation of the CCS in the LTP of the "El Cheapo". It is attached to a B- supply. I will keep an eye out for a suitable +/- supply transformer. Unfortunately I am in a 240 volt country, so a 120/120 isolation transformer is not my answer. I will be in Singapore next month, so will have a browse around Sim Lim for suitable.

Something like this may work Chinese R core transformer R26-42 50 watt, 115x2 primary, 0-50-55(0.45A) x 2 secondary. Wired with primaries in parallel would give about +/-75 volts, or primaries in series +/-150 volts @ about 200mA. Should also fit under chassis without too much trouble.

Now, the gain problem with the drive stage. I would like these mono blocks to have "standard" sensitivity. Should I re-think the driver tube and keep the topology, or do I need another drive stage? Perhaps revert to the original suggestion of using 12AT7 driver DC coupled to ECC99 LTP suggested by Eli?

Thanks again,

Chris
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Old 20th January 2009, 10:03 AM   #108
SY is offline SY  United States
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One other thing to note- the source follower driver will not need to be run in class A. As it swings negative, the tube is cut off and doesn't much care what the MOSFET is doing. So you can get away with a relatively low idle current, even though peak current will be much higher (on positive spwings).
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Old 26th January 2009, 02:32 PM   #109
hoka is offline hoka  Netherlands
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Has there been a post of the complete schematics for this amp?

I can't find it
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Old 23rd February 2009, 04:00 AM   #110
hoka is offline hoka  Netherlands
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any one for a schematics?

please...
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