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Old 30th June 2007, 04:00 PM   #1
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Default Bell & Howell amp

Given to me by a friend. One only and he kept the Tungsol 6550s.

Could be good transformers but I have questions about the circuit:

Is it pentode operation

Is 400V high for the screen

Can the 7199 be improved on

Any thoughts?

Thanks!
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Old 30th June 2007, 09:28 PM   #2
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It is a pentode mode circuit.

The O/P tube's screen grids run off the same supply the 7199 runs off. Don't worry about 6550/KT88 screen grids, as they are tough. Look at all the ultralinear O/P stages in use.

You can improve on the 7199 using separate bottles. It's difficult to better the performance of the EF86 as a pentode voltage amp. A high gm type is indicated for "concertina" phase splitter duty, as the exposure to slew limiting is reduced. The 6AB4 (1/2 a 12AT7) fits the requirement. The heater draw of an EF86 and a 6AB4 in combination is less than the draw of a 7199.
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Old 1st July 2007, 05:57 AM   #3
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Thanks Eli,

I thought it would be hard to replace the 7199 with a single tube but I found this Leslie schematic which uses a 12AU7 and has similar voltages and the pentode connection.

Is it workable for hifi use?
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Old 1st July 2007, 09:53 AM   #4
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No, that Leslie circuit is horrible. The phase splitter is very poor. Your original B&H circuit isn't too bad, although using a concertina to drive 6550 directly is pushing things a bit. Have a look at circuits like the Mullard 5-20 and the Williamson.
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Old 1st July 2007, 10:33 AM   #5
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I like the Mullard circuit idea, as it side steps problems in both the OEM and Williamson circuits. The OEM circuit is on the ragged edge of a "concertina" phase splitter's voltage swing capability. The Williamson circuit requires a very high quality O/P trafo to avoid being swamped by phase shifts.

A 12AT7/ECC81 as the LTP phase splitter and 1/2 a 12AX7/ECC83 as the voltage amplifier draw the same 450 mA. of heater current a 7199 draws and will provide all the gain needed. The requirement of using a high gm type to avoid slew limiting applies in both the LTP and "concertina" phase splitter cases.
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Old 1st July 2007, 10:56 AM   #6
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Thanks EC8010 and Eli.

How about copying the Michaelson and Austin TVA-1 which has 12AX7 input and 12AT7 phase splitter. Is it difficult to set up? My friend has one of these and I could have access to it for checking things.

By the way he kept the Tungsol 6550s for this amp but I thought it used KT88. Will 6550 work because that is what I would want to use?
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Old 1st July 2007, 01:00 PM   #7
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For the most part, 6550s and KT88s are interchangeable. T/S 6550s (NOS and reissue) have slightly lower max. ratings. So, an adjustment to the bias that makes the "idle" current smaller is in order.

I've attached a schematic of the "classic" LTP splitter. The ECC81/12AT7 is a better selection than the ECC83/12AX7, as previously explained. For good balance between the phases, the non-inverting triode's plate resistor should be slightly higher in value than the inverting triode's plate resistor. The 2 instances of Rl shown are, in fact, the grid leak resistors of the PP "finals". The AC load presented to each LTP section is (approx.) the parallel combination of the plate resistor and the grid leak resistor.

BTW, 200-220 V. on the plate and Ib = 3 mA. is a GOOD sounding operating point for the 'T7.
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