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RE: DB110G (Bogen) 1950s.

Posted by ivan_terrible (P) on July 2, 2024 at 02:56:07

In Reply to: DB110G (Bogen) 1950s. posted by ivan_terrible on December 26, 2023 at 11:20:29:

Now - Some months later.
Still working on improving the 1950s DB110.

Here is the FFT IMD trace using 6V6 in the original set up WITH NFB and tone controls - set FLAT, but now FIXED bias.

As you can see it's pretty dirty especially in the HF end.
A real MOUNTAIN of IMD sidebands showing up there.

It was impossible to get more than about 6W without it getting really dirty! (+It was worse with Cathode bias,- so much for Bogen 12W claims!)

This is the square wave response - very clean but maybe signs of ringing.

You won't ever get that from a DHT SE amp.

From this point I resolved to inject signal into the path directly beyond the tone controls - removing that as a source of distortion - which it turned out to make whenever treble or bass were boosted.

This lead me to believe the following stage - phase splitter and AF amp were doing something nasty, particularly to one phase.
That indicates inbalance and probably poor voltage feed.

In between times the 6k8 resistor in the HT line which fed the screen supply and then on the AF amp sections I reckoned was severely affecting it all because Pentodes draw a heck of a lot more screen current.

The resistor was halved in value which pulled the screen supply back up to 290V. Under full load this would drop to 282V.
As a result of that the AF amp section was pulled back up the 350V anode supply, resulting in an increase in current thru that section from about 750uA to more like 1m/a each.
The g1 leak resistor was changed from 100k to 200k fed from the bias setting pot which also relieved the ax7 phase splitter of AC load.

The final correct bias sets to -7.2V meaning those output pentodes have roughly 3x the gain of the 6V6.

That set the scene for yet another change...
Being as the output transformer is wired COM 4 8 16 (not 15 ohms) it means we can wire the 4 ohm tap DIRECT to ground NOT the COM wire.
The 4 ohm tap is therefore the CT of entire secondary winding.

This brings me to the final round of mods - which entails wiring/routing the output pentode cathodes direct thru the 2 halve secondaries, to give cathode feedback. NFB is therefore low impedance thru the cathodes, something which is only useable with high gain output valves.

We therefore get roughly 4V of CFB direct into the cathode circuit at full signal.
This meant removing the original NFB circuit altogether and bypassing the input valve cathode resistor with a capacitor.

Here is the result

IMD at 8.8V output:-
We can see roughly -20dB less IMD modulation products even with the roll off we got**

However we can get it to output 8.8V clean now (around 10W) so it's a big improvement.
In this case I was running 10 ohm on the 8ohm tap, so really 7.7W.
The anode to anode AC signal now is approaching clean 360V which is better. If we say 9kohm A-A that's up to 10W.

A significant rolloff at LF was detected with plenty of distortion at 20hz. Here is what it looks like - which fades away by 35hz.

Square wave now at full signal is like this (12W for the first time):-

Unfortunately now strong signs of inbalance which can be due to ONE weaker output valve, and sadly now a strong HF roll off, which we didn't expect.
No obvious square wave ringing or oscillation/instability.

I recorded roughly 0.5dB down to 50hz, with nothing less down to 30hz.
This is good.

However there is clearly a strong parasitic capacitive value being added in circuit now to drop -2db as low as 6khz. (It shows flat to 4khz).
At 15khz it becomes catastrophic at -8dB**

A valve amp with poor HF performance now who would have thought that?
(Need to cure that next).
Is it OPT or something else?

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