Hammond model E preamp question

[normende]

Hi all,

I would like to ask if any of you have an output transformer from the preamplifier inside a model E to tell me what are the values in ohms of the resistance measured in the primary coil and in each section of the secondary coil, and also if you have technical details about the transformer like impedance and number of turns as I have to find one that suits the specs because in my preamp it is missing.

Also I have a question related to the amplifier on a D20 tone cabinet.
It has 4 2A3s tubes and 2 56. I recapped it and changed all the resistors. Measuring +B and bias voltages I get 240V and -45V. I read on the 2A3 datasheet that it works with 300V and -62V. Are the voltages I measured within reasonable limits for the tubes to not be damaged? A technician replaced the rectifier tube with diodes and replaced the power transformer to lower the voltage but I think he exagerated maybe hehe.

Any info will be of a great help!
Thanks

[Drawbar Dave]

Contact forum member drummer55 for your preamp problems. He has spares for pre-war Hammonds and has supplied parts for my BC several times. Also have you downloaded a copy of the restored service manual? I can't post the link with my iPad but Google HSM495.

[tpappano]

That seems a little low for the B+ on the D20, I have a DX20 that I can measure this evening. I think the power transformer secondary is about 1000 volts center-tapped. The service manual does indicate -62 volts for grid bias but strangely, doesn't show what B+ actually is...

[Drawbar Dave]

I'm surprised you haven't picked up a Model E yet!

[tpappano]

You know, not for lack of trying 8) There was one that showed up in Southern Oklahoma last year for $200, but it was grabbed up before I could get a question about it answered. People selling it didn't even know what it was 8)

[David Anderson]

In terms of your 2A3 power amp, it's what's called a "back-bias" circuit, which is a slight variation on cathode bias. People get it confused with fixed bias where you have an independently-generated negative bias voltage supplied to the output tube grids.

In a back-bias design, the negative voltage for the 2A3 grids is created by the 2A3s themselves, via the cathode current they draw. With low plate voltage, plate current will be reduced, so the negative grid voltage will also be reduced. I think you are correct that whoever reduced the B+ voltage went too far. The amp will be running somewhat cold, but that won't damage it.

To put it another way, if your plate voltage is low, your 2A3 grid voltage will also be low.

[tpappano]

On my DX20 with line voltage of 118 VAC, I get 317 volts on the plates, -63 volts on the grids. With all original tubes the two 12" drivers speak "bigly" 8)
Elaborating on the grid bias a little, the bias voltage is developed by 250 milliamps flowing through the 250 ohm field coil of one of the speakers, which is in the negative leg of the power supply. That 250 ma is the sum of 60 ma from the 5100 ohm field coil of the other speaker, and 190 ma drawn by the 2A3s, all of which are connected to the positive leg of the power supply. Hammond seems to be running these pretty much as RCA specified for AB1 push-pull operation.

edit: looking at the tube characteristics, the tubes should be happy but will produce less output. My DX20 has a nice 'overdrive' effect, I suspect a benefit to the OP's system might be getting that 'overdrive' at a more tolerable sound pressure 8)

[normende]

Thanks to all for the comments! Interesting.

So it seems it is really running low in +B. There is nothing I can do about it unless I replace the power transformer... It was rewound with less turns that it needed.

Something I can do but could be a bit risky is adding a capacitor before the first choke. That will for sure rise the +B but it has some side effects too that I can´t recall now (I should look for the theory of operation of PI filters) and maybe because of that this configuration was not used in the original design.

Please if any of you can measure the resistance of the driver transformer or has some info about it will be great. I will check the service manual also.

[tpappano]

If the E uses the two-tube preamp, I can measure the output transformer ratio of the one in my BC this afternoon. You are correct that adding an input capacitor will raise the output voltage of the filter. A combination of input capacitor and series resistor should allow you to dial in the desired voltage if it goes too high with just the capacitor. The side effect is an increase in diode, transformer secondary current and heating, which may, or may not be a problem. Is the power transformer original, but re-wound?

[David Anderson]

I thought the Model E used a three-tube version of the early preamp with two 6J7 input pentodes and one 6C5 or 6J5 (half a 6SN7) output. But the output transformer is the same for all of them according to schematics: 17826-0. Interestingly, it seems there are some typos on the Model E Preamp schematics. One has "output transformer" and "filament transformer" flipped, while the other has the output transformer number mistyped with two numbers reversed. The second schematic in the manual, Figure 8, has the numbers correct; the first and third have errors.

To boost B+, you can start off by adding small amounts of capacitance, like 1uF, 2.2uF, 3.3uF, or 4.7uF film caps. You don't have to go all the way to a large 22 or 33uF reservoir capacitor.

[tpappano]

Here are the results of testing my BC preamp which, as David points out, should have the same transformer as your E:

Output transformer turns ratio is 10:1, which makes the impedance ratio 100:1
Actual output impedance of the preamplifier in operation, measured across G-G is 120 ohms,
so the impedance reflected back to the tube would be 12k ohms.

A possible substitute might be a small 120 volt to 12 volt center-tapped power transformer.

[normende]

You mean 6-0-6 or 12-0-12?

[David Anderson]

In certain parts of the country, a lot of two-tube preamps got replaced years ago by Trek II SSP-1 preamps, so there must be some original preamps sitting around out there that could be parts units.

Of course, the parts most likely to lead to a replacement are the transformers, but some techs prefer to replace rather than repair.

[normende]

Thanks David and Tpappano for having devoted time to look for info and measurements. Everything is really helpful.
The power transformer is the original one but has been rewound to adapt the AC voltage to solid state rectifiers instead of the 5U4. Unnecessary and expensive from my point of view but some people do it...

[tpappano]

You're welcome! Just curious, how does the D20 sound with the lower voltage, or do you need to get the preamp fixed before you can hear it?

[normende]

Someone borrowed me a driver transformer that is not exactly the one for this preamp but at least it lets me test the amp. So I cant tell what can be causing what I hear but volume is reduced and also and most important it seems like there is a distortion if I crank the volume up. Hard to explain because it is not a distortion heard when an input is being saturated with too much signal. It is different. Maybe saturation in the nucleus of the transformer in the preamp... So I need to play a bit with the pre before I can get a conclusion.

[normende]

I finally started trying caps in the entrance of the filter and 4.7uf was enough to raise the plate voltages from 242V to 290V. I will leave it like that as the tubes will have 10V marging to work a bit more relaxed. The bias voltage rised from -42V to -57V.
The effect was notorious. Much more clear. Not only more volume.

I still have to play with the preamp a bit trying another output transformer.
Something else I noticed... Filament voltage in the pre is 4.5V instead of 6.3V! Again another transformer rewound incorrectly.
In the amp the filament voltage was higher than 2.5V so I had to lower it with solid wire coils...