5F2 vs. 5f2A with 6g15 reverb

[Danelectron]

I recently completed a 6g15 replica reverb unit which sounded really good with a 5F2 replica I built a few months back. I was looking at the 5F2-A schematic and decided to modify the 5F2 to this 5F2-A. Not alot of difference, but the 5F2-A appeared to have a beefier power supply, and I thought I might gain some insight by comparing the two.

The 5F2-A sounds good also but with the 6G15 I get a bad hum. The 5F2 had a very, very faint buzz, but I chalked this up to the nature of tubes.

Question: Is there an obvious difference in these designs that is responsible for the hum in the 5f2-A?

Please see attached schematics.

I didn't have to modify much so the lead dress didn't change much so I'm thinking something else is causing this. FYI I live in an older house that has mostly ungrounded outlets so I use adapters.

Thanks for your responses. I always feel I should poke around further before posting, but with my lack of experience I think this forum is a much better guide.

[TD_Madden]

did you add the extra filter cap and adjust the others' values?

[Danelectron]

I replaced the first 8mf filter cap with a 16mf and another 16mf ahead of this just like the schematic. I removed the choke from the 5F2 and replaced it with a 10k 1W resistor. Added a 25mf/25v cap in series with the 470 resitor on the cathode of the 6V6.
I changed the tone pot to 1 meg. and relocated the 500pf cap as in the schematic and removed the 100k resistor from .02 coupling cap in the first stage.
I did change the grounding to pretty much match the layout schematic. I am suspecting this, but haven't opened it up yet.


What do you mean by "adjust the others' values" ?
Thanks for your attention.

[tubeswell]

In a 6G15 clone, you have to pay careful attention to grounding and transformers.

But also the reverb circuit is the most sensitive part of the amp. So you need to eliminate potential EMI coupling into the wet side. Ipso facto, tell me - if the mix control is rotated all the way to the 'dry' side, is there any hum in the amp?

[Danelectron]

if the mix control is rotated all the way to the 'dry' side, is there any hum in the amp?

Yes, the hum is present all the time. I used it with a different amp and it was less pronounced, but still there so I don't think the issue is with the 5F2-A. I tried plugging into a known grounded outlet, but no change. I moved circuit conductors around, and again no change.

Question: I grounded the RCA jacks grounding tabs to the chassis ground. Should they be isolated?

Tubeswell, Thanks for your attention, Dan

[tubeswell]

Okay so its quite possibly a grounding issue. How is the unit grounded? In this type of amp, you increase the probability of ground loops because you are plugging in to another amp that also has ground circuits, and both amps are plugged into the mains. (Back in the old days they never used to have 3-wire earthed mains - but lets not revisit those times just now). You need to minimise the number of overall ground-return points in the unit, and keep the ground-return paths separate as much as possible lading to those points. I would try re-doing the ground returns in the amp like in the article in the attachment (courtesy of R.G.).

Also, how long is your connecting chord between the reverb unit and your amp? It helps if this is as short as possible.

[Danelectron]

I'm using a single star ground for the circuit grounds. I ran a ground from the star for the input and output jacks. I read some where about isolating these from ground. Could this be my issue? I had originally run the supply ground to the star also, but per the article I moved it to the chassis where it comes through the grommet. This reduced the hum a little bit.

The cables from the reverb unit and tank are about 3 feet long. I will try to shorten these a bit.
I used an old network switch chassis for this unit so there is alot of distance between the transformers and they are also mounted at right angles to eachother.

I had reduced the mix and dwell at one point after these alterations and the hum faded away gradually. I had intended to just play through the hum for a bit so adjusted the levels down. After I noticed the quietI turned the pots back up to max and the hum returned to it's max level. It behaved as if the hum was bled off to ground after turning the pots down. I was short on time to mess with this so when I get back to it I am going to adjust the controls one at a time to see if one is more responsible than the other. I did try turning the mix all the way down, but no effect on the hum level.
Sorry I don't have more info at this time.
I am in the process of rewiring my house to update the receptacles to 3 prong, but I ran a grounded cord to my workroom to run these units in the meantime. When I was plugged into the ungrounded outlets the results were the same.

Thanks again for your attention. Hope your New Year went well!

[tubeswell]

If you are getting hum even with the mix on the dry side, my inclination is to think that you have one or more ground loop(s). So try doing the RG grounding scheme, including putting in isolating jacks. Also, when I was talking about short cable, I meant the cable between the unit and your amp.

Another thing has occurred to me tho' - what kind if rectification are you using, FW? or 1/2 wave? - And are you using 2 or more diodes in series? (You might want snubber caps in parallel with each diode (10nF 1-3kV). In my experience, these reverb units are somewhat noisier if 1N4XX7 diodes are used in series without snubber caps.

[Danelectron]

Tubeswell,
I'm using halfwave as in the schematic with 3 1n4007s. I guess I hadn't thought about the power supply. For full wave would I just use a standard bridge wave setup? I have a bunch of diodes.
How would I set up the snubber caps? I looked for some info and what I found will require some study, which I am going to do. Ordered Rudy Severns e-book. Looks interesting, probably more involved than I need for this, but I am finding it's true that no education is a waste of time.

Also, when I was talking about short cable, I meant the cable between the unit and your amp.

Oh,my misinterp. 10 foot instrument cable. Got yer point.

When I get back to it I will try shorter cable and set up full wave power supply.

Thanks a ton for putting on your thinking cap for me. I've been dusting off brain cells from early eighties electronics tech. I've got notes in my handwriting, but was that really me? It's slowly coming back with the help of this forum. Ive been doing electrical work for the last 25 years. A tech friend calls me "One wire". Now I get it.

[tubeswell]

I'm using halfwave as in the schematic with 3 1n4007s. I guess I hadn't thought about the power supply. For full wave would I just use a standard bridge wave setup?

Depends what your PT High-Voltage secondary looks like. If you have a High-Voltage winding with a center tap, then just make a standard full-wave rectifier (not bridge). You can then use the High Voltage centre tap with one or more reverse-biased 5W zeners in series with the CT to ground (cathode(s)/banded end(s) pointing to ground) to drop the B+ by a set amount. If you have to make a bridge rectifier, then you are probably best to put in a voltage divider after the filter to drop the B+ to around 300-350VDC.

I have a bunch of diodes.How would I set up the snubber caps?

1 x 1kV-3kV disc ceramic .01uF in parallel with each diode. See Merlin's website here for handy rectifier schematic pics.

The Valve Wizard

The Valve Wizard

[Danelectron]

Tubeswell,
Finally got back to this project. I set up a full wave bridge with snubber caps and using a 2 foot patch cable for now. The hum is tolerable though still present. Not much worse than some box effects I've used. The good thing, though, is that it really sounds good. The power supply modification really opened the thing up. Reverb to the point of echo. The tone control can really soften it up too. I've just begun to mess with it.
I am going to try to trace down the little hum that there is with a signal tracer. I get a slight buzz from my Strat that could be getting amplified to the point where I'm noticing it more.
Thanks for your help!

[tubeswell]

You're welcome. I'm interested to hear your 6g15 if you have some sound bytes. They sure are incredible sounding ain't they?

[MWJB]

You could be getting a ground loop as both the amp and the reverb tank are grounded via 3 prong cords. An isolation transformer on one or the other might help, or an audio ground lift adaptor inserted in the patch cable between the 2 devices.

[Danelectron]

Tubeswell,
Big thanks for your help with this. When my brother hears it he's gonna pi** all over himself. I'll see if I can put down some tracks. I have sonor, but haven't messed with it in quite a while. This reverb is really addicting. I will have to wear it out a bit cause right now I'm getting off on the "jammin' in a culvert" sound. Way out there. It can get real sharp or real mellow. Beats any combo I've had with on-board reverb. I built it in a disfunctional networking switch chassis that is rack mountable. I also have built a couple of small amps in these same chassis. I'm going to mount these in a rack that I have, which will be be very transportable. I will have to see if they will all function together in these close quarters without hum issues.

Which brings me to...

MWJB,
Where is some good info on setting up these isolation units? Which method is easier / more reliable? I don't have isolated RCA jacks in this unit. Actually quite the opposite, they are hard wire grounded to the chassis. The hum is really not bad at this point, but I would like to improve it as much as I can. It's kinda weird, I can move my hand around the tubes and different areas and make it hum louder. Seems like alot of inductance coming from the unit. Is this normal for this type of circuit? Tapping on the tubes seems very microphonic, but I chalked this up to the nature of this circuit. I have tried several tubes I have that are very stable in preamp circuits, but have the same issue in this unit. I haven't studied much on how this circuit functions or makes the tubes function. I was just jonesin' to get it together. So just ran at it and copied the schematic. Which explains all the questions.

Thanks again to you guys for your help, Dan