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DB110G (Bogen) 1950s.

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Posted on December 26, 2023 at 11:20:29
ivan_terrible
Industry Professional

Posts: 220
Location: Ivangorod
Joined: April 26, 2015



I posted some info on these nice old amps some years back.

Fact is, like most of the old school stuff they deliver rather disappointing power and dollops of distortion when pushed.
This doesn't go too well 70 yrs later.

I decided to have a go at uprating it - from the original cathode bias circuit and swop out the tired old 6V6s from years back.
Tired is pretty obvious when you see the voltage on the cathode bias resistor only around 14-15V when it's quoted up to 25V on the original factory papers.

Low current means higher voltage on the anodes so it was supposed to be 360 but could easily be anything from 380-390...as turned out to be the case.
Next step was to put in some of those great old westinghouse large anode 6V6, the ones with the look alike 7591.
That got the original setup working much better, but still anything over 5 watts is dirty.

 

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RE: DB110G (Bogen) 1950s., posted on December 26, 2023 at 11:53:02
ivan_terrible
Industry Professional

Posts: 220
Location: Ivangorod
Joined: April 26, 2015
 So, to cut a long story short I decided on another upgrade.
Short out the cathode resistor to ground make up a mini PSU with a 110v-10-0-10 transformer and inject fixed bias into it.

Here was the old small anode 6V




And here was the westi 7408




The mod is suprisingly easy, just put a 120k pot across the resulting 30-31V DC and there you go.
As you would expect all you really have to do is imitate the original bias point - around -21>-22V and you have it.

I really wanted to see how much difference it made.

Well.
The difference was truly astonishing.
The power output literally double giving good clean power to well over 10W

Now that was the sound I have been expecting to hear out of old 6V6 amps.
So, if you are still fighting with distortion forget cathode bias.
Get it fixed bias, and if you feel really ambitious drive the grids with cathode followers/mosfets all the way into AB2*.

The 6V6 was made to run in fixed bias AB1 and also AB2 up to 10V positive on the grid.
If you do that*, halve the output transformer reflected load by putting your 8 ohm speaker on the 15V tap, and the 6V6 will be putting out nr 25W.

That of course would mean changing the mains transformer and increasing the reservoir capacitors.
Talking of which the number ONE reason why these amps eventually die are the ancient twist grip dual section electrolytics.

Ask Jan Philipp Wüsten about getting some decent replacements of those.

 

RE: DB110G (Bogen) 1950s., posted on December 27, 2023 at 09:21:39
Triode_Kingdom
Audiophile

Posts: 10181
Location: Central Texas
Joined: September 24, 2006
 I won't use self bias (cathode bias) in any push-pull amplifier. Unless you're willing to bias the tubes nearly at Class A, crossover distortion sets in at higher output levels well before the limits of the tubes themselves.

Nice thing about driving the outputs with cathode followers is elimination of blocking distortion when peaks momentarily draw grid current. It's a lot of work and additional cost though.

 

RE: DB110G (Bogen) 1950s., posted on December 27, 2023 at 11:07:18
ivan_terrible
Industry Professional

Posts: 220
Location: Ivangorod
Joined: April 26, 2015



"I won't use self bias (cathode bias) in any push-pull amplifier".

Well that was what came with the amp, and for that matter, a large proportion of the 60s amps.
They do say, beam tetrodes only sound good when they are about to melt.
I reckon that's good advice.

I have run quite a lot of the old 6L6oids ie. 807/KT8
They do give best performance/lowest distortion when the anodes are glowing red in bits, or well over 85% of max Pa at idle.

The 6V6 (7408) Westinghouse valves are clearly better.
They look almost identical to the 7591, a far higher gain more linear component than the ancient 6V6 or the miniaturised 6BW6 and their clones.

Talking of this...I bought a load of 6BW6 cheap cos nobody wants them.
In the space for ONE octal (or 7C5 Loctal) you can fit a pair of 6BW6 so as to run PPP.
Actually quite a nice idea if that floats your boat.

 

RE: DB110G (Bogen) 1950s., posted on December 27, 2023 at 20:32:51
Triode_Kingdom
Audiophile

Posts: 10181
Location: Central Texas
Joined: September 24, 2006
 I'm a big fan of the 6V6, but being a cheapskate, I've been collecting sleeves of NOS 12V6 for a long time. Recently replaced the well-worn 7408s in my small dual-mono SEP with 12V Sylvania chrome domes. They're really superb. I also have a large stock of 6P3S-E and a couple dozen NOS 5881, so my mid-power DIY plans revolve mostly around 6L6 pentode designs. Those 6BW6s look interesting. I bought a large quantity of little 5686 8.5W pentodes some time ago. I'm using them in triode in a preamp project now, and hoping to use them in pentode push-pull in a future guitar amplifier. So many tubes, so little time!




 

RE: DB110G (Bogen) 1950s., posted on December 28, 2023 at 01:32:15
ivan_terrible
Industry Professional

Posts: 220
Location: Ivangorod
Joined: April 26, 2015
 Of course you can carry on collecting stuff for ever, but it's of little use.
Currently the loctal 7C5 go for peanuts.
I bought a large qty of Mcmurdo loctals to put them in if needed.
The 6L6 types can also be had for peanuts in loctal using the 254/255M series from STC - UK mil spec made.

I managed to start up a pile of them making an adaptor from UX to Loctal, and found out NOS is not so smart.
Production spread is huge, and some just arced on startup.
I did manage to make a matched quad like that and a few pairs, but then what to do with them? 807s/6L6 are known for hideous amounts of 3rd harmonic distortion.
The much vaunted KT66 was a distortion mush maker, but few seem to worry.

As for the Bogen I will convert it to EBL71, another superb piece of kit with much lower distortion than any 6V6 and 2-3x the gain. Those also can be had for peanuts, as they made zillions of them in E Europe for lousy noisy old AM radio sets.
They are supposed to run the same A-A load as the 6V6.
Look how low the anode voltage can drop without losing linearity!
It's going down to 20-40V and still conducting!





Now compare it with the dirty old 6V6oids..





Pentodes can reduce IMD, now you see why.
That particular one (with the 2 redundant diodes) was scarcely ever imagined in push-pull and has no info on fixed bias at all.....ie. probably works brilliantly without all that beam tetrode IMD, and heavy THD.

 

RE: DB110G (Bogen) 1950s., posted on April 20, 2024 at 06:23:51
ivan_terrible
Industry Professional

Posts: 220
Location: Ivangorod
Joined: April 26, 2015
 Interesting stuff again.

I ran the Westy 6V6s and went to a lot of trouble to check output.
The "westy" looks very much like a 7591, so the large anodes.

Basically anything over 7-7.5V output into 10 ohms and the amp starts to clip...
That's not much more 5.5-6W REAL...in FIXED bias already better than Cathode bias.
Far far away of course from the claimed output of 12W RMS.

I also measured IMD and as expected, no matter the output level, the 6V6 is pretty much a dirty horror story.
I started to install the "EBL" Pentodes in loctal sockets now and hopefully we get rid of IMD and have much lower THD.

I will post the results here.

 

RE: DB110G (Bogen) 1950s., posted on April 21, 2024 at 01:09:17
ivan_terrible
Industry Professional

Posts: 220
Location: Ivangorod
Joined: April 26, 2015
 Using pentodes instead of 6V6 beam tetrodes.




Not to give too much the game away here are the PDD2/UBL7x instead of 6V6.
As you can see, there is a fair difference in height.
You can also see, again so as not to give the game away...




The Loctal valve has the same construction exactly as the EL60 which was used as the first EL34- which became octal with the steel ring base.




Suffice to say the valve below right is a MINIATURE EL60/EL34 OK?
The gain of the PDD2/UBL7x at 9.5 mA/V is almost the same as the 11.0 mA/V EL37 which preceded the EL60, so the drive requirements are about 3x lower than the 6V6.
This means a considerable reduction in distortion in the AF amp/driver





It was quite a lot of work desoldering the octal sockets and replacing them with good s/h Mcmurdo loctal ones.
When I do this next time I use NEW Mcmurdo.



The NOS 6V6 were rated at 14W.

After seating the loctals properly they JUST clear the original Bogen cage




Next stage came to checking various things before messing with bias settings.
Now the pair of them installed



 

RE: DB110G (Bogen) 1950s., posted on April 21, 2024 at 06:03:16
ivan_terrible
Industry Professional

Posts: 220
Location: Ivangorod
Joined: April 26, 2015
 There is absolutely NO information on using this valve in anything other than cathode bias.
Ie. Fixed bias has never ever been done on the UBL/EBL output pentode.

To give some sort of an idea on this, the 6V6 needed roughly -20>-22V of bias at the amp's 350V HT.
After having a bridge rect go up in smoke from the previous bias circuit, I took pot luck and put a 12V DC supply directly into the bias circuit and started up the amp.
Suprise!
The HT was sitting at 400V+ and no sign of life from the amp.
The pentode is rated at Va 300V max, so I figured at this bias voltage the valves must be turned fully OFF.

Putting a trim pot across the bias supply and feeding the grids with roughly HALF the -12V of bias brought things back to life, and the HT dropped to 350V - still a tad high.

This is where life got interesting.
Wiggling the bias anything between 4-6.5V meant the idle cathode current could be trimmed to anything between 30-40m/a.
However, the HT would go up and down with whatever current you chose to idle at.

A funny situation happens where at -4V the anode current would do 40m/a with HT of 325V, but with a really fast dropping screen grid (thru a dropper resistor) to as low as 220V.

Max anode diss is rated at 11W, but this is 13..and no sign of red plate.
However as you can now understand you pull it back to -5V anode current is 36m/a but now at 335V..which is still 12W.

At -6V the screen grid (g2) is now rising to 250V and Anode of 344V with Ia of 32m/a which is Pa 11W.

then at -7V we have g2 at 260 and anode at 354V with 30m/a so 10.62W, so the right spot appears to about -7.2V where the Va is 355V and 265V on g2.
At this the valve is running 28.75m/a - just over 10W.

All these figures show the effect of increasing static load on the HT line as simply dropping particularly the screen grid supply.
In other words the usual 1950s/60s valve amp appears.

The anode dissipation of this pentode can be violated with no adverse effect at all.

The constant rising and falling of the HT supply under load, with the SAG of the valve rectifier is doing nothing less than varying the gain of the output valve and running out of current to supply both the anode and the screen grid (And of course the preceding AF amp and phase splitter).

Here we have yet again the classic case of the amplifier delivering scarcely HALF the rated power cleanly, simply because the PSU is not able to do it except on sharp transients.

This is the classic definition of a dynamic range COMPRESSOR as well as a special COLOURING equaliser with clipping occuring way before the output transformer is in saturation.

I will next compare the output before clipping of the IMD signature.
The pentode amp sounds cleaner, quite possibly because of the far better sensitivity of the output valves requiring only about 1/3 of the drive needed to full output.
+
The biggest improvement vital to do on the UBL/EBL version of this amp is to get improved stability of the screen supply.

 

RE: DB110G (Bogen) 1950s., posted on July 2, 2024 at 02:56:07
ivan_terrible
Industry Professional

Posts: 220
Location: Ivangorod
Joined: April 26, 2015
 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?

 

RE: DB110G (Bogen) 1950s., posted on July 2, 2024 at 08:28:21
ivan_terrible
Industry Professional

Posts: 220
Location: Ivangorod
Joined: April 26, 2015
 Some thoughts arrived since the last post... becoming a "blog"!

Conclusions:-

1/ The loctal x-BL71/etc is a great valve.
It stands severe violation - overvoltage, running flat out square wave testing, running the anode and screen at well over the recommended maximum diss - not a single sign of red plate or glowing screen grids like the OEM 6V6 btw.
The internal construction/glass seems well able to cope.

2/In detail Pa is quoted 11W. (plate dissipation).
Nothing bad happened static at 13.75W + 2W static on g2.
Under full load calculating back via the dropping resistor of 3k5, the screen showed a drop of 60V to 284V ie 17m/a.
17m/a is 4.9W for the 2 valves ie. 2.45W each.

The original figures for 6V6 state 13.5m/a at 285V for 2 valves max signal which is 3.85W or 1.92W each.
The 6V6 pulls just 4m/a at idle...0.5W ea.
This is to be expected with aligned grids.

3/ The drive requirements of a valve with gain 9.5ma/v compared with the original 6V6 of are very much lower.
It needs just 6.5v of drive to max compared with the 6V6 which needs 22.

In view of the much heavier screen grid currents of the pentode, the supply to that point needed reinforcement.

3/ CFB loop via the output transformer sec is around 6dB.
The original NFB loop gave about 15db of NFB.
The pentode has 50% lower THD (needs less NFB).

4/ OEM figures are WAY optimistic.
"With built in preamplifier giving 10 Watts output with only
0.65 % har monic distortion.
Frequency response: +/- 1 db, 15-50.000 cycles at 10 watts".

Valve manufacturer quotes max power output of 13.2W at 1.8% THD.
The 6V6 quotes 14W at 3.5% THD.

Distortion: was quoted 0.125%, 0.25%, and 0.65% harmonic at 1, 5, and 12 watts respectively; 14 watts peak
None were achievable here.

The IMD figures were no better originally between the 6V6-x-BL71 but after optimisation using CFB were up to 20dB better with the pentode.

This could not be checked with the 6V6 because the drive voltage requirements are more than 3x higher, so the effect of a 4v return loop would have substantially less effect to a -22V>-25V bias point.

I note the Bogen -25V voltage figures couldn't be met with the best 6V6 I could find.

FYI.
High Fidelity magazine, April 1955 quoted:-
" It is one of the best-sounding amplifiers we've heard in this general price class"
"Our checks on the power amplifier section showed low distortion at all power levels up to maximum, lower than in many amplifiers selling for a good deal more."

Our unit underperformed substantially.
After a lot of work we could get just short of 9V out clean into the load resistor.
This equates to around 10W, but with high losses of HF response.

Below is the much scratched out OEM diagram with loads of notes.
The next step is to test with a completely different (SCOTT) high quality transformer to see whether it's possible to improve on the BOGEN OPT frequency response.



 

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