Junkbox build

[joebog1]

I started a new build today!! I call it junkbox, but it should turn out nicely, ( I hope ).
The chassis I have had laying about for some years. I laid out the bits I'll use, and drilled it accordingly, slapped a coat of paint on it, inside and out.

Inside the chassis I paint pale grey flat, so that an auld farte like me can see and read colours. The outside is 50% flat, to go with the "modern trend".

The circuit is half plagiarized, and half invented. The power stage is from an Australian magazine, June 1954 in fact. Radio and Hobbies! I still have about 100 of them. It's a push pull "ten watter", or thereabouts, using 6BW6's driven by a 6AU6 phase splitter, which in turn derives signal from another 6AU6 pentode voltage amp. Output transformers are second hand 1962 made Japanese Tango straight push/pull design with 4, 8 and 16 ohm outputs.

The input stage is my own concoction using a 6BR7 pentode phono input stage, followed by a single voltage amplifier that does two things.
It supplies the volume control from the first triode, and drives a cathode follower, using the second triode, to TRY and drive the 2.178 inches of 8 gauge fencing wire that is otherwise known as a "digital input stage".

I only have vinyl to listen to, as I find the screeching treble of seedies isnt worth the money, or the ear pain.

There are no tone controls !! Controls are simply an input selector ( which I in fact don't need, as I only have my Garrard 401 to drive the amp.)(( maybe somebody will be silly enough to buy the amp ))
An Alps Black Satin balance control, an Alps Blue Velvet volume control, a neon pilot lamp, and a rotary on/off switch.

Power supply is a voltage doubler, with the power transformer derived from a National stereo amp/ radio with AM, FM (mono) and 3 short wave bands. No RF stage for the AM side. I "think" it used 6BM8,s or similar in PP so current supplied by the traffo should be sufficient for the power amp, and modest requirements needed by the phono stage.
Two 6.3 volt windings will be used, one of which will supply both power amps, probably floated above ground by about 20 volts to help cancel any hum. The other will be rectified to supply the phono stage. As yet I'm up in the air about using a resistor to drop back to 6.3 volts, or to use a three terminal rectifier.

I'll keep you posted ( provided the OPS allow my postings

Regards
Joe

[Ti Pwun]

Interesting. I imagine you could have issues with the chassis connections, given that it has been painted inside and out, but I look forward to seeing updates.

[joebog1]

There wont be any problem! After all there is ONLY one earth connection.
I also have a "Dremel" type tool with a small fine grinder that will remove the paint. I also have LOTS of ITL's or internal tooth lock washers.

The original article should anyone want it. Well, it's the circuit, I have the full article should you want it.

Joe

[Herald1360]

Curious way of specifying resistor values- either ohms or megohms with only experience or the parts list to elucidate. The .05 phase splitter loads seem to missing from the prts list too.

[Radio Wrangler]

Look out for the primary pole capacitor on the anode of the first pentode. It's the main stabilising influence on the loop. Also the shared cathode resistor for the output pair really requires quite a well matched pair of devices.

It isn't a particularly low distortion design, the tetrodes aren't used in ultra-linear mode or to use triodes, and there isnt going to be a huge amount of feedback because of stability concerns. Maybe it's something that has become favoured because it applies a fair bit of valve sound to the signal?

David

[joebog1]

Agreed to all the above.
I have built a few of these for guitar use, and they are quite good. ( using my own make of output transformer)
I will use some parts made of unobtainium to enhance stability .
I have another set of these output transformers, and they really are wonderful, if a tad ugly from hand stacked laminations. I havent had one of these to rewind, so I havent stripped one yet, but to my "eye" they are 9 section. Exactly how the secondaries are organised Im a tad unsure, as there are "taps" where various joins take place, that are not connected to the outside world. All wire guages are similar however.
It was common to use larger wire for the 4 ohm winding in Japanese transformers of this era. The laminations are 7 thou, which I guess enhances performance somewhat.

As far as the circuit goes, Radio and Hobbies was ( and still is) very highly regarded. All the writers/designers were engineers, some with vast amounts of experience. As digital stuff appeared, the quality went down, as did the projects. Radio and Hobbies ( began as Wireless Weekly in 1912 I "think" )
and later Radio Television and Hobbies designed some superb test equipment in their time. It changed its name to Electronics Australia and fizzled out about 20 years later.

Ill start adding nuts and bolts to the chassis in the next few days, and post pics as I go.

Thanks for the feedback all.

Joe

[Radio Wrangler]

I'm afraid that getting good stability in feedback loops isn't as easy as just adding unobtainium, it needs maths.

The capacitor on the anode of the first stage makes the gain of the main amplifier roll-off gently at 6 decibels per octave above a frequency set by the capacitance, the 200k resistor and the anode slope impedance of the pentode. The latter should be fairly high, so the rolloff should hit -3dB at about 8kHz. The aim of the rolloff is to get the amplifier strip gain down below unity below the frequency at which the output transformer and other sundry delays have built up to make 90 degrees or more phase shift. This is just the criterion for making a circuit which doesn't oscillate, we need to have a decent amount of safety margin in order to be sure the thing doesn't have an overshoot problem (Or a big peaky bump at the top of the frequency response) The more sensitive you are to screeching treble, the more you'll want to avoid this. The whole business of what can be achieved in a transformer-coupled valve amplifier comes down the the design of the transformer and the bandwidth it can achieve for low phase shift. It was careful transformer development that allowed H J Leak to use enough feedback to get down to that famous point one percent distortion.

I bought a distortion meter a while ago in a junk sale and got it going to give to my brother who builds guitar amplifiers... "What do I want this for?" "To make sure you're getting enough!" It was worth it for the joke.

David

[dave walsh]

Excellent-that joke works on several levels especially if your brother wasn't laughing.
Dave W

[Herald1360]

Would that be 11%, perhaps?

[joebog1]

A quick update, but several hours work.

More later!!
Joe

[frankmcvey]

Very nice job, Joe. What's your thinking behind the orientation of the valve bases? I ask because I'm planning a valve testing rig with 4 different bases and I'm considering the best orientation(s) to facilitate the wiring.

Cheers,

Frank

[joebog1]

Essentially I orient the bases so that the signal follows the circuit on paper.
By that I mean in straight lines. For instance, pin five (plate) of the first 6AU6 is connected to pin one (grid) of the second 6AU6 through its coupling cap, so pin five "points" at pin one.
I try to follow that "rule" as it keeps signal wiring very short. Power supply connections are essentially at ground, so I can tolerate slightly longer wiring, keeping in mind of course, that the resistor "body" is as close to the valve pin as is practical, for plate supply resistors, and bias ( cathode) resistors.
Gris resistors ( should they be required) are placed exactly at the grid pin, and a few mm above/away from chassis and "high current" components, such as plate resistors, or power supply decoupling resistors

The circuit is usually drawn in 90 degree plane, that is horizontal and vertical components, and I try placing resistors and capacitors as nearly as possible to follow that scheme. It makes circuit tracing very easy, and servicing ( should it be required) even easier! It does however make for slightly messier heater wiring, but as long as that is as far away from signal as is practical most people dont notice.

If you like, I try and follow standard RF wiring practice, and the above is just a long winded version of that.

Hope the explanation helps.

AS far as valve testing goes, it really doesnt matter all that much, as most valve testing is done using DC, but if your going to use dynamic signal testing, something along the lines Ive just explained could be used. Remembering of course that not all valves use the same pin connections.
This may play real havoc if you test "dumb" triodes ( read low gain ) and then try and wire in a very high gain pentode.

regards

Joe

[Radio Wrangler]

RF wiring practice is to put components like grid resistors, cathode resistors and capacitors and screen decouplers close to the valve pins and connected to the chassis very close by. Your earlier post says you only have to make a single connection to the chassis, so it's not clear what you mean.

I'd go for the local grounding to the chassis, short components, and short signal paths. It keeps things stable at high frequencies. The style of construction of things doesn't just have to be right for the frequencies it's wanted to work at, it also has to be right for the frequencies you don't want it to oscillate at. This is something which has bitten transistor circuit designers more than valve designers as device bandwidths have grown dramatically even for the same type number. The valve guys had enough trouble trying to get good enough screen decoupling to keep 4CX250s and their ilk from hooting.

David

[joebog1]

To explain my "single earth"!!.

I use star earth, i.e. a single tag very securely connected to the chassis that all earths are connected to. Its also the earth point for the main HT power supply, AND the mains earth from the power cord/IEC socket.

In the output stage, ( output stage means four vales, two 6AU6's and two 6BW6's) Im trying to construct the amp by using a single tie point. To that point Im hoping to have a layout that will earth the
1. 2 meg input resistor ( 2M2 today).
2. 100 ohm feedback divider from V1's cathode.
3. .47 uF decoupling cap from the screen supply.
4. 47K phase splitter resistor.
5. 100pF silver mica stability cap on the plate of V1.
6. 2 off, 470k grid resistors from the 6BW6's, plus two 100 uF electrolytics,
and two 510 ohm self bias resistors for the 6BW6,s. I may add a pot at
this point, so I can balance any pair of 6BW6's, the wiper becoming the
earth point. This will also mean decreasing the 510 ohm resistors.

Ideally the power supply decouplers, ( 16 uF electros in the circuit ) would also connect to this single point, BUT that may be a tad tricky.

This single point will then be connected at the exact point of the negative pole in the power supply, which is also the earth point of everything else.
I will use "heavy guage" wire, to connect this single tie point, to the negative supply pole.
I have dabbled with the idea of using the earth lug of the phono input sockets as the "single earth point", but that may prove more trouble than its worth.

Perhaps not ideal, but its as close as my feeble and lame thinking goes !!.
There are also some steel screens to be fitted,
1. Between the power supply, and the amp proper,
2. and between the phono stage, and the rest of the amp.

Wiring in between the various sections of the amp will pass through grommetted holes. The idea of feedthru caps for this amp is over the top.

I think that if comments wait untill I have posted the pics, will be beneficial to all of us, as commenting on every point, fault, other peoples ideas, or plain fundamental mistakes, can be rectified after the basic unit is constructed!!

He said, cap in hand

regards
Joe

[frankmcvey]

Ah, so! Thanks for the explanation in post 12, Joe - makes sense. As you mention, my little rig will just be for DC testing/ matching etc, so I don't have the same concerns as you re signal paths. Also, as you say, pin wiring varies between different valves, so that precludes taking any special precautions with routing heater wiring, so my main routing concern will be neatness.

I'm enjoying watching your progress - thanks for posting.

Cheers,

Frank

[joebog1]

I have done a few more things!!.

Most of the smaller components are in, as is the heater wiring and major components of the power supply. I still have to add bias C's and R's to the 6BW6's, and the stability cap on pin 5 of the first 6AU6.

I have so far, managed to maintain my single point earth!! This is the first lug of the large tagstrip in between the two 6BW6's. There is still a power supply decoupler to be added ( 15 uF ).

Yes, it looks in the photos to be a short circuit waiting to happen, but all components are well apart.

Ill keep you posted..

Joe

[Diabolical Artificer]

Tidy work Joe- the joint on the 4k7 resistor looks a bit iffy though as do a few others ( HTR wiring ). Sorry to teach you how to suck eggs but I think your iron isn't hot enough or you need to leave it on the joint a bit longer and use a tad more solder. Hope your not over the UK soon, don't want to get thumped. : )

Interesting project. Andy.

[joebog1]

I think what you are trying to point out is that there is very little solder on the aforementioned joints.
Thats because its very difficult to poke another wire into the joint !!
The joints that are finished have fully filled holes in the tag, socket pin or transformer tag. The half finished joints will be completed when they have all connections made to them. As it is I have to sometimes remove excess solder with my solder sucker to add bits/wires
My soldering iron is a Hakko 936 with a 2.5mm spade tip running @ 480 C
The solder is alas 60/40 rather than 50/50 but I dont often produce dry joints.

best regards

Joe

[Diabolical Artificer]

I did wonder if that was the reason there wasn't much solder on the 4k7 joint ( 2nd pic ) the equivalent of tacking something when welding. I was also worried a bit about the joints on the htr pins on the bottom valve socket. Heater wiring is always tricky the holes on the sockets are never big enough.

I probably use too much solder sometimes and guess soldering technique to some extent is a personal thing. Hope my comments came over as an observation and not a critisism.

Andy.

[joebog1]

Not at all Andy !!!!

er hmm perhaps my aussie speak doesnt always work!
I know the yanks absolutely hate my use of English

YET !! I hate the word goddamn, and Im not even religious

BUT back to subject! I will always take suggestions, be they good or bad.
After all, its my main reason for being here. Im am a "very average" engineer who specialised very early into a very narrow niche.
Thanks for the comments
Regards again
Joe