• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Which EL34/KT88 PP projects should I consider?

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6

6A3sUMMER

Member
Joined 2016
pieter t
You are correct.
The specs for cathode to filament voltage on the ECC88 are 50V (or 90V) on system 1 (pins 1, 2, 3); and 130V (or 150V) on system 2 (pins 6, 7, 8), depending on which old data sheet specs you read.

But I wonder 2 things:
How many amp builders pay attention and connect system 2 as the top triode in the cascode?
How many manufacturers of current production use separate filament to cathode
construction materials and techniques to make section 2 take more voltage?
For that matter, how many new old stock tubes were built to meet the differences in system 1 and system 2 cathode to filament Voltage specs?
 
V

VictoriaGuy

Member
Joined 2008
6A3sUMMER said:
Amp # 1
At turn on, the 6GK5 can have large grid current equal to the current source value, if the B-supply powers up before the B+ plate supply.
Or, on a ‘hot start’ after a very brief power outage (filaments still warm, but B- comes up before B+ comes up).
Wow! way to much grid current (can potentially damage the tube).
You will have to be sure about the order that the 2 power supplies power up.

The grid current can come out the volume 100k input pot, especially if the pot is turned all the way up. Might be a disaster for some solid state preamps, or CD output ICs.

If the power supplies do come up in the wrong order, higher resistance grid stopper resistors could limit the grid current.........



Amp # 2 (Allen Wright amps)
These are much more complex amps and power supplies.

Again, the amp can draw grid current under a wrong order of B+ and B-power supply turn-on,
and/or ‘hot’ start B+ and B- power recovery times and order.

But the current source supply could rise to 1.4 times -12V.
So, grid stoppers that are large enough to limit the current may also cause high frequency roll off.
Click to expand...

Eli Duttman said:
The "El Cheapo" I'm associated with is similar to Poinz's "Musical Machine". I took slowing B- rise down into account. A hybrid bridge rectifier using an indirectly heated type achieves the need. :yes:
Click to expand...

It sounds like this is a real problem. I thought a lot of amps had been built to both the Pointz and Allen Wright designs? Have these types of damage happened to those builds?
 
6

6A3sUMMER

Member
Joined 2016
Eli Duttman,
Thanks for the idea for how to solve the B- timing problem.
I will have to get my hands on some 6AL5 tubes (I only have 6.3V filament windings).
The cathode to filament voltage rating is plenty to work with the differential voltages of filament supplies that are elevated to + 60V DHT filament self bias, and moderate B- voltage supplies.
 
V

VictoriaGuy

Member
Joined 2008
6A3sUMMER said:
The 6V6 are using an 11k plate to plate transformer.
Those are not likely on old Fisher amps, and probably not in your parts box.
Click to expand...
Thanks for your very complete comments; I appreciate them. Plenty of things that I didn't realize....more learning required!
Again my apology for linking the wrong schematic. That 11k Bogen OT was used when Pointz rebuilt an amp for a friend/customer. He actually specifies an OT in the 8k range as OK for the 6V6 amps- easier to find, I think.
If those diodes are 100V zener, that is the wrong symbol for a zener.
And if they are a zener, they are drawn in the wrong polarity. The cathode always goes to the more positive voltage (towards the 330 Ohm resistor).
If those are ‘regular’ diodes on the 6V6 screen, don’t use them; just connect directly across where they would have been.
Click to expand...
I think they are probably regular diodes as they don't appear in most of the Musical Machine versions. I've added that to my notebook for this project!
 
R

rhone

Member
Joined 2005
VictoriaGuy said:
Rhone-
Did you build both the PP1C and the PP2C, or a combination of the two?
Did you use the Wright 'SuperReg' or a similar one you built?
Click to expand...
I originally built the PP1C. Then I read a post of Allen's where he recommended the PP2C driver stage for 300Bs, as in his DPA300B. So, onward and upwards, I converted the input stage to the 6H30's of the PP2C and changed the output stage to cheap 300B's. All on the one stereo chassis.
However due to space limitation the PS was too rudimentary, the cheap 300Bs difficult to balance and I wasn't happy with all the heat from the cathode resistors. I've moved on to an outboard power supply and NOS 6B4G output tubes which were cheaper than even the basic 2nd hand Chinese 300Bs I previously used. But I did learn a lot and that was really the whole point!
All iterations have used the "Salas SSHV" shunt regulator on the inputs.
I'm tempted to change to VR tubes with a CCS mainly because I have never used them and so that's something else to learn about. :)
 
6

6A3sUMMER

Member
Joined 2016
VictoriaGuy,
I made a major mistake in my original message, the 6V6 amp was Triode wired, I stated that it was Pentode wired. Triode mode has a moderate damping factor (not the Pentode's low damping factor that I mentioned). Loudspeaker impedance variations versus frequency affect triode mode less.

The EL34 machine is very much the same as the 6V6 machine.

One difference is that on the EL34, pin1 suppressor grid is paralleled with the screen grid.
I believe the suppressor is normally (perhaps correctly) connected to the cathode (for EL34 this has to be done externally, it is not connected inside the tube as most pentodes and beam power tubes do). That makes a difference too, if you are tube rolling from say a 6L6GC or KT66, etc., and then changing to an EL34, remember to connect pin 1.

A project years ago, called the Randall amp, did tie the screen and suppressor grids together. So, when I did an EL34 amp, I tried it both ways, but I did not see any
difference in performance.

I still have to be convinced that connecting a diode in series with the screen supply does any significant good.

The effect of grid current at the input stage is only sometimes a problem. Part of the problem is when extremely fine wire grids are used. I knew of someone who ruined a 12AX7 in that kind of instance.
I like the 12AL5 (6AL5) solution that was shown by Eli Duttman in this thread.
 
V

VictoriaGuy

Member
Joined 2008
nerdorama said:
I built the Musical Machine with 6B4G output tubes into a 5k transformer. It sounds really good. Haven't had any problems.
John
Click to expand...
Interesting.

Which version of the MM did you start with, and what changes did you make to the schematic (voltages, etc.)?

What is the power output of your 6B4G MM?

Do you know what the THD of your amp is, and how does it compare to the THD of the 6V6 or EL34 versions of the MM?

Thanks.
 
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