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Bogen DB212 phase-inverter

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I am working on a Bogen DB212 integrated amp. I notice the phase-splitter is the split-load/concertina type but it's a little unusual here.

What is the advantage of arrange it this way? Input pentode's screen-grid is fed from the cathode of the triode. It seems unnecessarily complicated. Is the balance good? Can I rework it into a more traditional topology? Is it mainly to get more gain and still able to bias the input pentode properly?

Bear in mind this "integrated" amp does NOT have a line stage so having as much gain at the amp's input stage is necessary.

Simplified schematic of amp section only:
An externally hosted image should be here but it was not working when we last tested it.

Full schematic:
R24 and C3 form a DC servo loop to keep the splitter at an optimum operating voltage. No direct effect on gain. Wavebourn uses this scheme in his amps. R28/R29 provides some actual Positive Fdbk to the input tube from the splitter, besides being part of the attenuator for the the global feedback.
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Well it's somewhat complicated.Since R24 is decoupled by C3 it is in parallel with R28 therefore the concertina sees R24//R28=77k at the cathode.So R27 should have the same value.At the series 56,68,82,100 they took the best match, 82k.
The R28 normaly ands at ground level but here the is a R29 in between.That gives positive feedback and a high open loop gain.Now there is at the same time an overall negative feedback via R30 to keep the hole thing onder control.
If you put 1M2 in parallel with R27 it becomes 77k.Could be an improvement.
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FWIW, I have some observations/comments.

It seems the low level mag. preamp uses AC heating. Bogen was not kidding about 7025s. In this day and age, that means the Sovtek 12AX7LPS.

The 150 mA. EZ81/6CA4 is working at its very limit, maybe over its limit. Perhaps reworking the power supply to cLC and SS rectified is in order.

The O/P "iron" looks undersized. Perhaps switching to triode mode "finals" would yield better bandwidth. Triode wired 6V6s sound very nice.

The 7199 has gone the way of the Dodo (extinct). Rewire the sockets to take tubes from the 6GH8/6BL8/6U8 bunch. Stock up too, as all of the other desirable tri/pents are disappearing too. :(

Tape head I/Ps are passe and switches in mV. level signal lines are problematic. I find myself repeatedly suggesting that units of this time frame and character be reworked to hard wire a single set of RCA females to a permanently RIAA configured mag. preamp and all other I/P's be line level. An under $5 Lorlin rotary switch from Mouser makes the modification quite easy.
Preamp tube heaters are DC powered, as part of a common cathode bias 'tail' of the output stages, so common 12AX7 should be fine rather than special 7025.

So, Bogen, like several other manufacturers, used the "Cheap Charlie" method for DC phono section heaters. :down: The phono section heater current is modulated by the O/P signal. YUCK! :mad: IMO, yet another necessary modification is the construction of a proper, regulated, DC heater supply, for the phono section tubes.

I an not impressed by this particular Bogen effort. The "corner cutting" is (IMO) all too obvious.

The combined idle plus signal currents from both output stages pass through the preamp heaters and a bypass cap. That RC filter has a 20Hz corner frequency. There would be negligible signal current in the bypass cap during class A operation. If the bypass cap is located with the output stage, then negligible output stage signal current goes near the preamp location.

The worst-case signal through the heaters will only occur at high output power levels, and would be orders of magnitude less than a typical AC heater voltage magnitude.

Audio signals will be increasingly attenuated with frequency due to the bypass cap. At worst, any audio signal would couple over to the preamp cathodes - those cathodes are grounded, so there is no heater-cathode resistance/capacitance injection mechanism.

If anything, that heater powering configuration needs a fail-safe path (eg. from a pulled input stage valve) to somewhat protect C11.
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The heater voltage changes, but only when output stage is pushed beyond class A operation limits, and then the change is very well filtered/attenuated and proportional to the extent that class B is entered.

So the effective AC voltage appearing at the heater is likely to be two orders of magnitude below what would be experienced with mains AC heater powering, and buried below the high signal level (ie. there is no hum from that mechanism at idle with your ear up to the speaker).

How much of that heater AC voltage or current then ingresses in to the audio signal then depends on a variety of issues.

One issue is when there is a cathode impedance to ground (as per normal cathode bias circuitry) in the first one-two input stages - in that Bogen, there is no cathode bias circuit (it uses grid-leak bias), so there is no hum ingress from that path.
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