Home Made TRF

[Neil Purling]

I built the Electron Coupled TRF radio earlier this year with a 6J5 and 6V6 for the audio stage, and I am now considering further improvements.
I have a problem with a certain level of hum, it's not really powerful and the caps in the PSU are a pair of new 47Uf @ 450V electrolytics.
Is it a issue that there is still RF getting through to the audio stage?
I was considering getting another chassis box and rebuilding the PSU in it so it isn't in too close a proximity to the signal stages.
How can I improve the PSU? Are there optimum values for the two capacitors?

[Ed_Dinning]

Hi Neil, there are ways to calculate the size of the smoothing caps but values above the optimim will not usually cause problems. Your values should be OK.
Try the effect of a good paper/ polyester cap across the e-caps, say 0.22uF. If it is modulation hum, due to RF on the HT line this should cure it.
It must be remembered that the impedance of E-caps increases above the audio range, whereas the impedance of a non-polarised cap will be small at RF.

Ed

[paulsherwin]

The output transformer will sometimes pick up hum from the mains transformer. If this is the case it will still hum even with the valves removed

Paul

[Neil Purling]

I pulled all the signal valves apart from the rect: All quiet
6V6 (next to HT transformer) & rect: Quiet
Adding the 6J5 triode produced the hum.
The audio amplifier was therefore complete, but there was no signal input because the EF39 was removed.
The hum varied with the position of the volume control.

An image shows the layout of the radio.

If I added the 0.22 uf capacitor, would I place it beyond point B in the diagram, between the HT line & chassis?

[Neil Purling]

Oh, and by the way I have made sure that it is not the PC or the ceiling light, which is one of those energy saving tiny flourescent bulbs. Check made at 7:30am when nobody else was awake.

[Skywave]

Hi Neil.

Reading the cct. description in your first Post, (and subsequent Posts), am I correct in deducing that the stage config. is:

EF39 detector (pentode) with reaction,
6J5 AF amp. (triode),
AF gain control,
6V6 AF O/P (pentode) to O/P transformer,
F/W rect. (double diode) for the PSU?

Knowing the config. will help towards locating the source / cause.

Al.

[Neil Purling]

Here's the digram of the detector
https://www.vintage-radio.net/forum/...8&d=1141415399

The amplifier
https://www.vintage-radio.net/forum/...9&d=1141415399

I re-wound the coil to omit the 100 turn winding. Theres simply three turns of wire for aerial coupling without loading the detector.
The amplifier is culled from Radio for Boys by E N Bradley. The 6J5 is used in place of the triode part of a EBC33 with no change in component values.
The 6V6 cathode resistor was changed for 470R so the 6V6 isn't running practically at full bore (as it was before).

[Skywave]

Hi Neil.

Thanks for that info.

Your hum problem. What immediately springs to mind is that as far as AF is concerned, the anode of V1 is at high impedance. Therefore, the slightest trace of 50 Hz / 100 Hz pick-up on this feed up to the 0.01 uF at the AF gain control should be in screened cable - ideally earthed at one end.
Ditto the feed from the wiper of the AF gain control to the control grid of the 6J5. All this circuitry is at high impedance - and therefore prone to a.c. pick-up.
This fits in with your reported condition that the hum level is dependent on the setting of the AF gain control.

I would also be happier to see some form of RF filtering at the anode of V1 to keep RF out of this AF feed.

Hope this helps.

Al.

[murphyv310]

Try feeding the screen of the 6V6 with a 2.2k resistor decoupled with 4.7mf, also the ht feed to the preamp triode should be fed with a 4.7 k ohm decoupled by a 10mf cap this should cure the fault. Have you used screened leads and earthed the body of the volume control if not this can cause pick up, also only earth one end of any screened leads preferably at the volume control end as you may have a hum loop. Are you using a good smoothing choke preferably ex equipment resistor smoothing is not so good.
Trevor

[Neil Purling]

I thought that the presence of hum was due to inadequate smoothing or the proximity of the transformer to the small signal stages.
When member Heatercathodeshort made the schoolboy breadboard radio the HT transformer is at the other end of the radio from the tuning coil and the small signal stages.

If I re-make the PSU in a seperate chassis: What would be the most appropriate values for the capacitirs and for the suggested choke?

If I am getting RF passing into the amplifier how do I eliminate it?

I will alter the PSU first, then apply previous suggestions for amendments to the amplifier.

[Neil Purling]

Amongst my accumulation of junk,sorry stockpile of useful components I came across a choke. All it says is "WEYRAD", the maker I presume.
I have no idea of the value but assume it is for a PSU.
Its effect is to apparently lower the pitch of the hum. I may be going soft but can this change to the PSU have any effect on the audio?

[6AL5W-Martin]

http://budoclub-biedenkopf.de/51j4/psu.jpg

[Neil Purling]

I will make the changes in the previous post.
I am also considering a rebuild to include a Jackson 36:1/6:1 reduction drive & dial unit.
If I keep the same coil as before and use a 365pf tuning capacitor will this make any change to the behaviour of the tuned circuit? (Apart from the reduction in coverage of longer wave-lengths that is).

I am presently using a EF39, and was wondering about the effect of using the other valves, like EF80 and EF91. I wondered if the HT supply to the detector should be kept to a minimum. What would be the ideal for each of the 3 valves, and what is the effect of too high a HT voltage in this area of the set?

[Neil Purling]

Replying to my own post, for the benefit of members.
HT on anode of the EF39 is 75 volts. How can I make the way it comes to oscillate a more gradual. With the EF91 it seemed more 'knife edge'.

My hum problem seems to relate to the volume pot, I think there's a poor contact.
Anyone got a good 1Meg logrithmic ex-equipment pot?
I don't have the jewellers tools to perform surgery on the one I have now.
There are also some issues with the variable capacitor, but that is due for replacement anyway.
Will there be any difference in the performance of the electron-coupled detector if a 365pf var capacitor is used rather than the 500pf example at present?

[Skywave]

Originally Posted by Neil Purling

My hum problem seems to relate to the volume pot, I think there's a poor contact.

Have you tried the earlier suggestions made here that involved using screened wire to feed the the top of the AF gain pot and from its slider to the grid of the next valve?

Secondly - earlier you posted some above chassis pics. of the radio.
A few pics. of beneath the chassis might help us help you.

Al.

[Neil Purling]

I have re-made the connection to the volume pot with screened wire, which has made no difference.
I definitely think I have a bad pot. I don't think much of these modern pots you get in Maplins. The thing is there isnt a high voltage on the track of the pot. Anyone got a good, used 1M pot? Alternatively I could substitute a pair of fixed resistors.

Anyone had any thoughts on the effect of changing the variable capacitor?That isn't too good either.

[Skywave]

Originally Posted by Neil Purling

I have re-made the connection to the volume pot with screened wire, which has made no difference.
I definitely think I have a bad pot. I don't think much of these modern pots you get in Maplins. The thing is there isnt a high voltage on the track of the pot.

Re: screened wire: good. Presumably, you also fitted screened wire from the pot slider as well. Also make sure that the "bottom" of the pot. has a solid, low resistance connection to chassis.

Yes, the pot. could be faulty - but I can't, off-hand, see how it could cause the hum. (But I'm prepared to learn! ) If the pot. has a metal case, try to arrange that this case is securely earthed as well.

As regards "high voltage on the track of the pot", good! You certainly don't want HT at that point!

Regret that I can't help with your other needs.

Al.

[Neil Purling]

Screened wire from the EF39 anode to top of pot, screened wire from pot slider to grid coupler of 6J5.
When I re-organise the set I'll replace the pot AND earth the metal case.
Hopefully I can source a better pot.

[Skywave]

Originally Posted by Neil Purling

Screened wire from the EF39 anode to top of pot, screened wire from pot slider to grid coupler of 6J5.

Good.

Just another thought . . .

Upon re-examining the cct. diag., there just may be excessive 100Hz ripple on the HT to the detector. (Unlikely with the value of your HT smoothing caps., but you never know! Thinks: earth loops, etc.) So, try fitting some local HT decoupling to that stage: something like HT+ to 22k-ohms and from this to the 220k anode resistor and the resistor at the top of the reaction control pot. Take the junction of these 3 resistors to a 8uF cap. with the -ve side to chassis. If the hum disappears, the HT line had insufficient smoothing for the detector.

Only takes a few minutes to try this - and if it works - job done!

BTW If you have a 500k-ohm pot to hand, you could always fit that as a replacement for the suspect 1 Meg-ohm pot.

Al / G8DLH

[Neil Purling]

I changed the circuit of the detector by utilising a new Jackson Type 0 365pf tuning capacitor, rather than the previous 500pf one.
Reaction is applied to the second grid of the EF39.
Could the change to the tuned circuit mentioned above change the way the circuit behaves in respect of the amount of reaction required?