Hi All

I have an 11 20 plum a" K " model I believe it is... ..fitted with a 6AN7 and a 6M5.

What I found upon examination was that it had been worked on partly in the past and had had 2 extra 8 μF Paralleled across the original 8 μF B+ cap, there was one cap replacement on the 6B8G (mustard).

I have done the usual recap of the Electro's and removed the extra 2 caps ...both the 30 μF and 8μF were gone...and I replaced all the wax jobs (leaving the mustard) also some resistors that were reading a bit high, 2 X 1 Megs.

The 6M5 Socket was damaged, thought sure you could use it I decided it should be replaced
(after carefully tracing how it was hooked up, and lots of Photos).

That was a bit of a job but done now (if not perfectly vertical) I replace the 50 K and * 500 K associated with that Valve to a 47K 470K (*that was out).

B+ is sitting at 230~235 VDC across the 8 μF (now 10uf) I haven't looked at the Back Bias yet (resistors measure in spec, thought 35 ohm is more like 39 ohm).

So to the pondering Question?

It does work, thought the Volume isn't as powerful as the other Plum using the EL 33 output tube and in comparison to the little nipper running a 6M5 output its rather weak thought no real distortions
(the circuits are different thought so humm )

I haven't tested the 6M5, thought I can switch it out I suppose

Anyway.... anyone know if this something thats is common in the K version with 6M5, that the audio is weaker power wise or is it more likely I have a real problem to chase ?

My hearing lose dont help matters either & Id just rather not go chasing butterflies if there is none.

If anyone likes I can send Brad a redrawn Schematic to upload here

This is based on the original 1120 with the 6AN7 and 6M5 in circuit as per this radio's configuration but differences may exist...and probably do)

Thank you All As always any help very much appreciated.

PS Brad

I probably am an habitual offender, Grammer and correctness of format with writing was never something I did well at at school

I will try to lift my game thought.

If you have another set with a working 6M5, subbing is the best reality check. There is always the risk of a wrong component, or miss wire, however I see a step missing.

Most of the manufacturers recommend the re-alignment of the set once you have replaced RF components, as that can throw it off. You require an accurate signal generator.

The backbias is not the same, nor one resistor, in many of the variants. I would expect a bit under7V from CT to chassis for a 6M5 albeit that data is not clear: CT is neg chassis positive for that measurement.

One of the most common causes of hum in a back biased set is the connection of the negative of the first filter cap to chassis (Ground) the same as the second one, when it should go to the CT.

Marc

Hi Marc

Thank you For your reply

Yes I changed out the 6M5 with a NOS one (once I found it), seems it is the cause & based on other discussions here about that Valve... thought it looked OK in this instance it seems it pretty much up holds the image of "a lot you meet will be stuffed"

The RF IF alignment will need to be done, Yes most defiantly (as it needs to be done to the other one too) but saving that for another post on alignment as I do have a few questions in that regard.

I do have frequency counter (EA 500 MHz,just got to find it) and got a crappy Eico 324 sig Gen but it does work and a Cro (thought I've never used it for a Valve radio alignment)

AM Signals here arnt the best and I really need to string up an ant but I was pretty sure from the few I've done and the ones I have to guess the signal its self wasn't the cause of low volume.
Thought yes its easy to think that could be the case and that thought did cross my mind at first

Edited after re reading what you said Marc
---
The Back Bias "if memory Serves" follows the schematic But I will recheck that point it wouldn't be the first mistake I've made
(oh and I was only commenting on that one resistor I didnt mean it was the back bias)
---
OK just checked
30 μF Negative to Center Tap of HT winding of transformer from this point to a voltage divider of 150 ohm then 35 ohm to gnd, VD point feeds 1Meg resistor

CT (and Cap neg) to 300 K resistor that then goes to NC pin 9 of 6M5 that is a hook up point for capacitor from IF and 50k resistor which then feeds pin 2 of 6M5

It follows the schematic and this part is (or rather was) original, it still had red paint marks on it so I know no one had stuffed it up
(I'll double check that against the photos thought to be sure)

back to my original part of the post
--
Thanks to your invaluable input Marc on my first post here I now know what the "Back Bias" is about.
So I am now very aware to check that its connected properly, but I do wonder how many good radios
have been junked cause someone didn't know how that circuit works.

I have other work to do inside this one but it is now at a functioning stage by the looks of it so I will now do what needs to be done to finish it then Alignment.

Just additional.

I am not sure if the 2 X 8 μF were where someone attempted to fix this or someone at an earlier stage just decided to up the 8μF to 24 uf.

May suspicion is it was an attempted repair that would never have worked properly if both the originals (30 μF and 8 uf) were stuffed (but perhaps they were border line).

The Electro caps were old ones dating from the period and a little after so its hard to pin point when this mod was done especially if someone had a junk box of old caps ..they did test OK capacitance wise, the add ons caps I mean but thats no true measure of them.

The 6X5 tested OK for H~K leakage but seem a bit low emission wise, so I may just change that out for a Nos one (I guess that poor tube took a beating with dry caps and add ones).

The 6B8G thought physically is a bit battered, as in cracked base and probably breaking away for the glass envelope...so although its probably OK electrically I may change that too.

Of course there is no guarantee's that what I put in will last, still I've no desire to be opening it once done so Nos and cross fingers I guess.

both would be ok for testing purposes (once I glue the 6B8G).

Anyway I'll think on that.

It did have lovely case no broken parts (wonder of wonders) and still had the bakerlite shine once cleaned up so it wasnt a bad score I guess at the end of the Day.

Kriesler 11-20 Circuit Diagram.

Just bear in mind I am not perfect and there probably is errors as it was based on the original schematic, thought I am sure the Valve hook ups are correct for the 6AN7 and 6M5.

I must point out that the 6AN7 drawing is rather all over the place in the diagram and so not too conducive to the flow of operation, but does go where it needs to as per the physical Circuit

I probably should have reversed the Valve layout but anyway it was a first redraw and provided I haven't had a"Doh" moment is correct to the electrical connections.

Sadly the original component numbers and parts are blurry.
(it would take some time to correct that so I just left them)

Please again if anyone spots a glaring error let me know.

Component Values may vary, especially in regard to cap values for mica's and inductors, these are from the original
circuit as listed (best I can make out anyway).

Thanks Brad much appreciated, I was rushing to get this emailed this morning before I went to make my donation to the TAB...very Sad about that horse thought.

Thank you

The important bit with the 6B8 is that the base is less important than the envelope; Araldite & tape have been invented. You use the super strength one and use the tape to hold it (and Adhesive) in place while it reacts & solidifies. You can even have the radio running with a cooler tube like that, while you watch it set.

There is a trap with frequency counters in that many do not appreciate modulated signal (tone on) can cause count errors. It really does not matter how crappy the sig gen is, as long as it is stable & produces a clean signal. Many generators need a cap in series with their output to reduce signal & provide DC blocking.

I have made a "Black box" that allows the audio to be hooked to the CRO sync. has the attenuators and all plugs that are required to neatly hook it all together and avoid wires running everywhere.

The CRO is not any easy apparatus to understand, but I find them to be the swiftest device in conjunction with a sig gen to find lost signal. It is much better for aligning as you can see what is coming through & if there is distortion, it will be seen and you can then use it to help find the source CRO will also see if an oscillator is working.

Marc

I have made a "Black box" that allows the audio to be hooked to the CRO sync. has the attenuators and all plugs that are required to neatly hook it all together and avoid wires running everywhere.

Sounds ideal. I reckon a photo and details in the Workshops & Tools section would be appreciated.

Again Thank you Marc

Yes I was looking today and thinking about just gluing as you have posted about before on, I got some slow setting Araldite (thought this stuff seems to be a little hard to come by with everybody selling fast setting stuff.)

Id rather keep the original in there, thought I do have replacement Nos ones (untested by me as yet)
This one does have a Shield for the 6B8 (sadly no shield come with the other wreck)

Thanks for the tip on the Frequency Counter, I was at the lest aware of that point on modulation and yes it being stable...thought I have yet to actually put it through its paces on stability..it is a tube based job as you'd know

I've been reading a lot and read a lot of your posts both here and at the other forum along with others
still its the "doing" part thats the learning curve I suppose

"avoid wires running everywhere."

I'll second GTC on that Marc, could be quite a useful project if you care to share at some point.

Yeah I haven't really used a Cro for Radios, actually its something that although I learned about radio I never had much to do with repair wise even thought I was into 27 MHz and 477MHz. I never got around to becoming an Amateur Operator thought... Morse was still a requirement then and thought I tired to learn it I just never stuck to the leaning of it sadly.

I suppose I really need to go attend one of those HRSA open workshop days or something, Anyway Thanks again Guys much appreciated.

There really is not much to this black box as it's main function is to direct traffic by taking the outputs from the sig gen & distributing them, to mainly BNC so that they can couple via coax so there is no signal radiating everywhere.

The sig gen I have is a clone of an LSG-11 and I have one of those and it's original box. I think there a four sig gens here and I do not need that many, all are valve. What I find bizarre is the low voltage B+ used, as it puts the operating point of one valve right on its knee curve and that is a recipe for distortion.
The main difference between the LSG & Clone is that one has a 0.001 cap before its attenuator & that causes the counter grief, when it cannot get enough input. That is another reason for the black box.

It puts the counter before the cap that has to be added to the clone. It would be nice to have a generator with a counter that does not need some of this. But when the OP is around 0.1V, or more & you need micro volts & DC blocking, you need to attenuate.

The idea with valve stuff is to leave it long enough to warm up & stabilise. We had sensitive German & Swiss instruments (among many) in a Lab I worked in and despite these having VR tubes & hot wire Barretter's they still recommended an hour to stabilise.


Marc

Photo & circuit of attenuator sent to Brad & post awaits as per request.

Marc

Thanks Marc

Yes one of the things as you say, warm up and letting them stabilise

Theres a few Caps that I want to replace in the 324...its had most caps replaced but it had ceramics put in I think not safety caps on it mains side so I want to replace those...
(it just not the done thing today to have them there and Safety Caps should be in place I reckon regardless)

I'll have better look at you other post now, but yes I do think shielding the signal is a good idea as you what the signal were you want to put it, not being picked up spuriously by everything..or chances of that happening

Thanks again