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Old 17th July 2014, 08:46 AM   #1
6J7 is offline 6J7
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Default 8552 finals ?

Hi,

I'd like to use 8552 (12V version of 6146B) as P-P pentode with a 4K/10% CFB OT. After reading this article I wonder if this is a good idea ? or isn't it a problem for audio use ?

What do you think about that ?
Thanks
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Old 17th July 2014, 11:40 AM   #2
DF96 is offline DF96  England
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Fine, provided you know enough about mid-power RF amplifiers to tame all parasitics. Remember, the valve does not know that you want it to stop amplifying above 25kHz. If it thinks you have built a nice UHF oscillator for it to live in then it will happily oscillate for you, however much you shout at it to stop!
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Old 20th July 2014, 08:06 AM   #3
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Thanks DF96.

Quote:
Fine, provided you know enough about mid-power RF amplifiers to tame all parasitics.
Unfortunately..

What do you recommend ? 10R inductive wirewound resistor on each plate near topcap and 100-200R for g2 ?

Or ferrite bead ? I found this:

Code:
Specification:
Ferrite type	bead
Impedance at 100MHz	85Ω
Mounting	THT
Impedance at 25MHz	35Ω
Core material	Ni-Zn ferromagnetic
Inductor application	RF interference suppression
Ferrite series	FRH
Dimension	8mm x3.5mm
Leads	axial
I don't know if it's well suited or not.
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Old 20th July 2014, 03:36 PM   #4
DF96 is offline DF96  England
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I have never built an RF amp with a 6146 so I have no practical experience in taming them. I have a few valves in my stock, so one day I might try.
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Old 21st July 2014, 07:44 AM   #5
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You are building an audio amp, not a RF amp, so it shouldn't be a problem.
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Old 21st July 2014, 09:22 AM   #6
DF96 is offline DF96  England
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How do you tell a 6146 that it should only amplify audio?
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Old 21st July 2014, 09:47 AM   #7
AJT is offline AJT  Philippines
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6J7, if you can get a copy of the 1960 ARRL handbook, there is a modulator amplifier using the 6146....
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Old 21st July 2014, 12:30 PM   #8
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I've not found the 6146 to be any more difficult than any other beam tube in terms of parasitic oscillation. The 6L6, for example, is pretty much an 807, which is rated for RF power amp use up to 125MHz.

Just make sure you put grid stopper resistors on G1 and G2, right on the tube socket. I don't think you need any additional precautions.

The RCA datasheets show operating conditions for push-pull class AB1 and AB2 operation.

I do think it works a lot better in A(B)2 operation than in A(B)1, but that's another issue...

Pete
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Old 21st July 2014, 12:30 PM   #9
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Quote:
How do you tell a 6146 that it should only amplify audio?
How do you tell a 6L6 type to amplify only audio? The same techniques work with the 6146. Many people realize that the venerable 807 has exactly the same internal parts as the 6L6GA that it was made from. The 807 has some extra shielding around the lead outs in the base to reduce RF radiation.

The 6146 actually behaves more like an early sweep tube than a 6L6 type, or any of the other typical audio tubes. Many people have built successful audio amps with the 6146 type including myself. Treat them like any other sweep tube.

DO NOT ABUSE the screen grid rating! This generally rules out triode and UL circuits. Ampeg used the 6146A in the early SVT 300 watt bass guitar amp. They blew up, and some even caught on fire. They quickly switched over to the 6550, and their reputation was saved. The 6146's was not. This amp has given the 6146 a bad rep in the audio world. They fed the screen grid far too much voltage which can kill the tube (or ANY sweep tube) when the amp is over driven. Do guitar players overdrive their amps??????

The 6146....and just about any other power tube, can amplify up to 50 MHz at full ratings. The ratings for "non RF" tubes must be reduced above 50 MHz. The 6146 is good up to about 175 MHz. This is important to RF guys, but not audio guys.

To keep the 6146 stable apply usual clean layout techniques, use grid stoppers on G1 and G2. Keep the plate leads (OPT wires) short and routed away from the driver circuitry. I will also slip a few ferrite beads over the leads on the G1 stopper resistor, and often the plate cap wire. Many sweep tubes are far worse in the parasitic oscillation department than the 6146 types due to the higher Gm.

The article you referenced does point out the differences between the different flavors of "6146" in RF amps. This is very important in RF amps because we need to amplify one range of RF signals while preventing operation (intended or otherwise) at another RF frequency. The internal capacitances of a tube, transistor, mosfet, or IC chip are part of the RF tuned circuit. If these parameters are changed, the circuit is no longer "tuned" and bad things happen.

The grid stoppers and ferrite beads are what tells the tube to amplify only audio by forming low pass filters in the low RF range. You want clean SHORT layout between these elements and the tube socket to avoid creating unintentional resonances in the RF range. A long wire IS an inductor, and the grid of a tube IS a capacitor. An inductor and a capacitor make a resonant circuit. You can't change the grids, but you MUST keep the wires short enough so that any unintentional resonances fall above the frequency where the active gain device (tube) ceases to have gain.

How do I know this? I just retired from a 41 year career as a transmitter designer for Motorola.
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Old 3rd August 2014, 04:08 PM   #10
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Thank you very much for your replies.

Only 2 stages RC coupled for this amp, so AB1

As I have a lot of 6AC7/6J4, I'll try them for the driver. This tube seems to be rarely used, is there any specific reason or people doesn't like metal tubes ?
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