Hiwatt Custom 50W Amp excessive rectifier buzz in output

[nevetslab]

I have a Hiwatt Custom 50W Amp SSD504 on the bench, and when I first looked at it early last week, I found excessive rectifier commutation noise coupled into the input stages. I have since replace the 1N4007 1A 800V rectifiers with UF5408 3A/1kv rectifiers, along with changing the bias rectifier to a UF4007 from a 1N4007, just to be thorough. It also was missing one of the ballast resistors, and not having yet received the incoming 1W carbon film resistors from Farnell UK, I swapped out the 220k/1W on the first filter stage, replacing those with 100k/2W MO, which gave me a spare 220k 1W resistor for the ballast resistor across the next series-coupled cap array.



Yesterday, having set the Custom 100W amp aside, I moved this back to the bench to see if I had made any progress with the rectifier buzz on the output. Those changes didn't make any difference. I read thru numerous posts on Rectifier Noise in our archives, found one on a Fender AB763 deluxe build, and read the post R.G. had submitted, advising the HT C/T lead MUST BE connected to the bottom of the first Filter Stage, rather than just to Chassis Ground. In this chassis, the HT fuse is in series with the C/T, and was wired to chassis ground just below the fuse holder (bottom view). I disconnected that, and laid in an insulated wire from the HT fuse terminal to the bottom of the filter cap array where it ties to chassis ground. Checked the results, but still made no difference.



I next looked at the input grounding, and compared that to the Custom 100W amp. I don't have this coupled rectifier buzz on the Custom 100W amp...at least orders of magnitude lower. Looking at the two amps' input grounding, maybe I could improve upon this.



This slight wiring revision on the Custom 50W now looks close to that of the 100W amp. I did remove the single-hole ground lug and replaced it with a #6 dual-hole solder lug, and first tied the pots ground buss to it, then that from the input jacks, and finally that from the two cathode resistors of the input stage tube, reducing the loop area from where I started. Though I had doubts this would make any significant change. It made no difference....I still have the same degree of rectifier buzz.

The power tubes installed in this amp are KT77's, matched, set for 35mA plate current. Just to see if it made any difference, I replaced those with a matched pair of EL34's, same plate current. That too made no difference. Below is an inside view of the full chassis.



I don't have any high voltage ceramic caps that could be used across the HT rectifier diodes at present. If that sounds like a sensible plan, what value would you recommend? 10nF/1KV? 0.1uF/1KV? On the cabinet, there is adhesive-backed aluminum ground plane that gets tied in once the chassis mtg screws are anchored. That hasn't cured this either.

I don't see the rectifier buzz on the power supply lines, having looked from HT1 thru HT5 supply potentials. I've attached the schematic, with Pin numbers and voltages present below

[Jon Snell]

Put it back as it should be, 1N4007 NOT UF5408 if it is switching noise you are getting.
Here is the original scematic of the power supply.

UF5408s cause untold problems due to the type of semiconductor used.

Edit;
The original BYX94 was a REC53A (1250v PIV 1.25A) from RS Components, many years ago obsolete as is the common replacement CGP15 (1450v 1.5A) which is also obslolete.
If smoothing hum, check the state of the 220uF tank capacitors and the balance of the output valves.
Not a good idea to use 100k balance resistors, they get hot in time and because they are mounted on the capacitor legs, conduct their heat into the capacitors. I know it is only half a Watt of heat but it all adds up to unreliability.

[Helmholtz]

The spikes in your scope pic look like rectifier induced heater hum/hash.
See scope pic and explanation in Merlin's book, page 76.
You probably have a clipped heater voltage and the heater-cathode capacitance couples the differentiated heater voltage to the cathode(s).

A humdinger often helps (lift CT of heater winding).

Does the noise reduce when yo pull V1?
As the heater-cathode capacitance of tubes varies, a different input tube might give some improvement.

[nevetslab]

The spikes in your scope pic look like rectifier induced heater hum/hash.
See scope pic and explanation in Merlin's book, page 76.
You probably have a clipped heater voltage and the heater-cathode capacitance couples the differentiated heater voltage to the cathode(s).

A humdinger often helps (lift CT of heater winding).

Does the noise reduce when yo pull V1?
As the heater-cathode capacitance of tubes varies, a different input tube might give some improvement.

I DO INDEED have clipped heater voltage. I don't have Merlin's book (Designing Valve Preamp for Guitar and Bass, Second edition), but looked on his website under Power Supplies, and read thru his Heater Supply section. When I remove V1, the coupled noise drops over an order of magnitude, and looked the same as turning both Normal and Brilliant Ch Volume Pots fully CCW. Changing tubes made no difference, and trying a 12AY7 in V1 position did reduce the magnitude of the main noise spikes, but also added more hash. I haven't yet tried lifting the CT, which on both the Custom 100W and the Custom 50W is tied to the first Power tube socket nearest the power xfmr, at Pin 1/Pin 8, rather than directly to the adjacent Ground terminal. I could try lifting that connection and installing a 10 ohm 2-5W Power Resistor to see if that changes things. I could also try installing a 100 ohm/5W Humdinger in place of the C/T.

I haven't tried any of the other 12AX7/ECC83 pulls I have on hand...only tried the J/J tubes. I'll see what I have and if there's any improvement with a different mfgr's tube, though most of the pulls were removed for excess noise or microphonics.



I also looked for my magnetic field probe that I got with a mumetal shielding kit, and was certain it's here in one of my supply boxes with other probes and cables, but, didn't find it. So, just for starters, I wound a small 10-turn coil of insulated 28AWG wire, but had to reduce it to 8 turns to fit thru the Tek A6302 Current Probe jaws, and used it as a makeshift field pickup coil. I see I need a lot more turns to go that route....don't have any small AWG magnet wire on hand. I did look around the chassis with that probe, not finding anything in the vicinity of V1 or the input jacks, volume pots, but looking adjacent to the heater wires where they attach to the first power tube socket (nearest the power xfmr), I did find conduction spikes:



In reading thru Merlin's Power Supply Section, Heater Supplies, I see the favored Heater Wiring distribution used by Hiwatt isn't the most favored wire routing, since the two heater wires route the wires around the top/bottom of the tube sockets, rather than thru the middle. I think that can be overlooked, as this is their method selected, and in the Custom 100W Amp, I'm NOT getting this severe rectifier-coupled noise spikes. I'd have to pull that chassis back out to look further.

As far as changing the rectifier diodes back from the newly-installed UF5408's to the 1N4007's as were in place, not seeing any difference with either ones in place, I don't think that's an issue. Current documents I have from Hiwatt indicate they've changed to the UF5408 rectifiers.

The use of the 100k ballast resistors is a temporary move....and will be changed back to 220k 1W as soon as my parts order from Farnell UK arrives, probably next week. The leads are actually bent over and down, so there's a good 1/2" of lead length used to get them to fit the spacing of the snap-in caps.

Power supply hum doesn't appear to be an issue.

[Helmholtz]

Your second scope pic is identical to the picture in Merlin's book.
THe spikes are produced by the small but very steep voltage drops at the right side of the clipped tops. These contain lots of HF..

As said, the simplest cure typically is a humdinger.

[nevetslab]

Your second scope pic is identical to the picture in Merlin's book.
THe spikes are produced by the small but very steep voltage drops at the right side of the clipped tops. These contain lots of HF..

As said, the simplest cure typically is a humdinger.

I've just been going thru a collection of pulled tubes....12AX7's from a variety of sources....Sovtek, TAD, PM, Ruby, Groove Tubes. I found several of the Groove Tube 12AX7's NOT having the conduction spikes, but as all these are pulls, I was finding many being microphonic. All my new tubes here in the shop are J/J, but we have others at the Guitar Dept, so.....seeing as there might be a solution on this amp by just selecting a tube other than the J/J's....that plus seeing what I can do as far as mounting a humdinger could be the solution. Installing a small resistor in series with the C/T is also easier than mounting the humdinger pot.

More to follow on this. Thanks for your input!

[Helmholtz]

. Installing a small resistor in series with the C/T is also easier than mounting the humdinger pot.

I don't think adding a resistor will make much difference as it doesn't shift the balance.

A small 470R/100mW trimpot won't require special mounting.

[g1]

I DO INDEED have clipped heater voltage. I don't have Merlin's book ([FONT=Arial]Designing Valve Preamp for Guitar and Bass, Second edition), but looked on his website under Power Supplies, and read thru his Heater Supply section.

The excerpt is here: https://books.google.ca/books?id=9kM...H0Q6AF6BAgjEAM

[nevetslab]

I don't think adding a resistor will make much difference as it doesn't shift the balance.

A small 470R/100mW trimpot won't require special mounting.

This morning, having thought it over in how I'd mount the humdinger pot, I removed the outside power tube socket hole cover plate nearest the power xfmr (an aluminum plate), marked the center, punched a 5/16" hole to accept the 100 ohm 5W WW Hum Balance Pot I have a small stash of, and mounted that just below where the heater wires tie into V5 power tube. Wired up the twisted pair & wiper to attach across V5 Pins 2 & 7, disconnected the C/T wire, capped it off, and tied the wiper to the ground terminal where the CT had been.



With that mounted, I connected my AC-coupled X10 scope probe to the heater connection at V5 Pin 2, and turned up the volume pots for Normal and Brilliant Channels to 3 o'clock position, with the Master at max, and all the tone controls at full CCW, as I've been doing. I then went thru numerous tubes as I had done at the end of yesterday's investigation, where I had found several having very little RF spikes....though were noisy and microphonic. It was enough to verify Helmholt's suggestion of trying other tubes to find some with different heater-cathode capacitance, and see how the addition of the humdinger lets me 'dial in' and suitable tube. That same batch of tubes from yesterday wasn't yielding the same results, though after turning off the florescent bench task light and adding a shield plate to partially close up the open chassis (standing vertical with the probe connected), it showed I should be able to dial in a suitable input tube.

I drove over to the Guitar Dept down the street, though all I could find there were eight new Mesa 12AX7 tubes, and brought those back. Here's the results of that effort, new tubes and best balance dialed in on the humdinger:



Two different Mesa 12AX7 tubes displayed above. I copied/pasted the text from the first photos into the second one, instead of having to retype all that verbage. I wasn't able to use any of the new J/J ECC83 tubes here. All the tubes installed in this preamp section are J/J, while the power tubes are Matched J/J KT77's. I'll have to order more ECC83/12AX7 tubes....Grove Tubes' appeared to have far less of the RF coupling, as did the TAD 12AX7 and PM E83CC tubes.

g1.....thanks for the link to the Merlin's book. I'll order a copy of that today to add to my library.

I must say, THIS has been an interesting adventure in trying to get an amp quiet. Thanks a ton, gentlemen!!

[Helmholtz]

So you could lower the spikes by maybe a factor of 6?

[nevetslab]

Yeah....about that much. I now need to move the Custom 100W amp back to the bench to compare these two amps. With this Custom 50W, turning up the tone controls, I'm still seeing those conduction spikes, but not really hearing them...nothing like before. What I AM hearing is the chassis being microphonic, now that the chassis is back into the cabinet. Figures. I didn't go thru the other seven Mesa tubes I brought over to screen for that parameter. So, not done yet. At least I've made progress.

[Helmholtz]

The shape of the distorted heater voltage is influenced by the PT (leakage inductance and primary DCR) as well as the current pulses charging the reservoir cap.
I would expect a PT having lower leakage inductance to cause lower spikes.

[nevetslab]

I found today, after checking the Custom 100W amp, which was nice and quiet, while the Custom 50W amp appeared to be microphonic.....that was due to a jumper left out on the LINK jack that couples both the Normal and Brilliant Channels together. The Normal on the Link Jack was missing the jumper to Ground! So, both channels were not muted/shorted to ground, so tapping on anything sounded wrong.



Soldered that into place, NOW the amp is nice and quiet. Production omissions, I guess. Onward to the 20W 'Little Rig' amp

[nevetslab]

The following week, after finally getting to the Hiwatt Little Rig, I found it had the same nasty buzz on the Brilliant Ch like I had on the Custom 50W amp.



I found similar clipping on the heater voltage, though I forgot to capture images of it. I removed the blank hole plate in the preamp tube array, punched a mounting hole to receive
a hum balance pot, wired that up and then went thru the preamp tube selection, again finding the J/J ECC83S tube wasn't allowing me to null out the RF spikes while changed to a different 12AX7A (from our Mesa tube collection), that again restored order and minimized the buzz with tweaking the balance pot just added.



My TE Connectivity 1W and 2W Carbon Film Resistor order just arrived from Farnell UK/Newark, so I'll now be able to remove the temporary 100k/2W ballast resistors I had placed on the Custom 100W and Custom 50W amp heads. While I'm making those changes, I wanted to look at the heater voltage on the Custom 100W amp, as that amp did NOT have this rectifier buzz problem that the Custom 50W and the Little Rig heads had.

I must also say, this IS the first time I've had this sort of problem with the J/J ECC83S preamp tubes, and their presumed higher cathode/heater capacitance that seemed to be letting this rectifier noise in on these two Hiwatt Amps. Is this a common problem, and I've just NOT been catching it all these years, or is it perhaps an issue with the current rating specified in the products' power xfmr, not allowing for sufficient current to run the heaters in the power xfmr? I have had hum problems with Vox AC30 Power Xfmrs, and on two of our many aging AC30/6TB amps, adding a separate heater xfmr. (Which had the side effect of increasing the already-too-high plate voltage for the EL84 tubes!).

On a separate issue, I was disappointed to find the use of the flimsy spring-wire hold-down clamps used on the EL84 tubes in this amp.



I looked around to see if there was any better hold-down clamps on the market for clamping EL84's, 7189/7189A's than the common flimsy spring clamps seen here? I saw something from Stewmac....Belton 50703. Found same at www.amplifiedparts.com, with a couple pdf drawings. Seems these require different tube socket mounting.



s-h145_EL84_Tube mounting-1.pdf

s-h145_EL84_Tube mounting-2.pdf

Anyone have any experience with these clamps, or find something similar, allowing chassis surface-mounted tube sockets for the EL84 type power tubes?

[Pedro Vecino]

I usually use them in combos. For example in the Pro / Blues Junior when they have lost the original system accompanied by tube dampers. Also in some others.
However, on amp heads I use these different ones.