18Watt.com

Latest voltages marked on the left with a line. Little significant difference. As I said before I have a pair of output tubes giving only 20mv on the grid but ive paired them with leakier ones so combined giving 40-60mv.

AC varies here.

Sent from my SM-J600FN using Tapatalk

To recap what I think I've read so far...

You probably regret making a 36W instead of an 18W. You were considering a PPIMV to reduce the volume/headroom.

2 of your 4 power tubes are likely faulty.

You're unhappy with the distortion sound from the amp.

I can't read the voltages, the image is too small.

The only reasonable advice I can give you is to take the 2 GOOD tubes, and ditch the 2 bad tubes. Rewire the amp as an 18W and get the voltages where they need to be. Listen to it. It will probably sound a lot closer to what you want, and give you the dirt you want, with more usable volume levels.

This amp drives the power tubes to get that sweet distortion sound. If you're power tubes are problematic, you'll never get a good sound.

Stupid tapatalk app shrunk the photo.

Ok I think I've finally made it. Josh diagnosis of bad ac power was totally right, unfortunately the power conditioner does f all. The intermittent awful distortion was the motor of the washing machine coming on and off...

Sorted through until I had 4 power tubes I was happy with. Very happy with the spiky cleans with the full 36 Watts.

Fitted the vvr to the whole amp, to be honest I'm not bothered about chasing super low volume distortion, I just play cleaner when I want it quieter. I like the vvr a lot though.

Also added the pp-se mod which is basically a ppmv for one side of the power section. Love this mod too, reduces overall volume quite a lot and changes the flavour in a tweedy early gibson feel .

Still have slight microphonic issues with the chassis in the combo cab but I've no more time to investigate or build boxes so it's just sat on top for the moment...

Nice! Glad to see you made progress.

Did you use a filtered inlet, or just a filtered power strip? I like to use both.

You should post updated voltages and see what your power tube dissipation is now.

It's possible some of your tubes just happen to be microphonic, but it's also possible that the voltages are increasing a tubes tendency towards microphonics. You can tap each tube with a chopstick and see if any of them respond louder or differently than the others. With the amp on so you can hear it.

Thanks josh.

Honestly I'm surprised I still have any tubes left after the hours already spent tapping and listening...

I used just the filter strip. I'll order a filtered inlet next time I order anything.

I'll post new voltages but I doubt they've changed much from my last post a while ago. Haven't done much since I finally noticed that the timing of the nasty distortion was very similar to that of the washing machine turning and pausing...

Still got a sibilant metallic nastiness getting into the sound. Nothing in these voltages causing me trouble is there?

I'm assuming there's nothing suspicious in the above voltages? At this point considering getting an amp tech to look at it...

chaliapin wrote: ↑

Mon 09/18/23 6:08 am

I'm assuming there's nothing suspicious in the above voltages? At this point considering getting an amp tech to look at it...

What are your cathode resistor values for the power tubes? 150Ω? Have you calculated your dissipation? If it's 150Ω, then you should be okay there.

V1 plate voltages are low.

Share a sample of the audio problem. Descriptions aren't often enough.

What are your power transformer specs?

Thanks josh. Cathode resistor is 150 yes.

Pt is a danbury pt04. I can't find anything on the net but I think Barry at ampmaker used to supply them in his kits. Attached the sheet I got with it plus tea stains...

I tried to record a couple of clips but the volume distorts my phone mic. I'll try to sort something else out. It's not as bad when the chassis is on top of the combo rather than bolted in. It's in the treble, sounds like sitar-like fret buzz but it isn't.

I'm thinking could be lead dress, hopefully borrowing a looper pedal so I can chopstick and listen at the same time.

Ok this is another phone recording. The vvr is on 8, I know that's not what you want but I'm getting the same high pitched ringing at full power but the phone recording falls to pieces at that volume. I switched both pre valves, no change.

https://vimeo.com/865899860

Holding the tubes doesn't get rid of the high pitched ringing which you can hear after the guitar begins to fade.

Does the same noise come through with other guitars?

Have you ruled out any ground loops? Have you tried that with the VVR completely bypassed?

Have you tried different power tubes?

Ruby mod?

Thanks josh.

Ringing seems slightly less with strat but still there. More prominent with vvr up full. I'll try it bypassed entirely.

Can you link me to some ground loop information please? Google didn't give me anything very specific.

I'm still thinking it could be my poor ac power, I need to try turning off everything which could be causing dirty power.

Oh and the ruby mod is already on. I feel like I'm going a bit mad listening to it but it isn't far from sounding right/ good to my ears, just the ringing and slightly unmusical distortion?

I would remove the VVR from the circuit to rule it out.

I would also make sure your outlet is grounded properly, and that there are no external appliances plugged in on the same circuit, and preferably nothing on that can interfere.

I would also try different tubes.

In the recording, it's hard for me to hear it, it sounds like the guitar. It could obviously be something else though.

There's also the possibility that your lead dress could be causing a parasitic oscillation that would negatively impact the distortion sound and cause weird noise like that, but again, you need to work towards ruling things out. I guess post some updated photos of the build and we can see if anything stands out.

Managed a spare 5 minutes so Here's strat neck pickup only, amp volume 8 vvr full. Guitar volume gradually increasing. No funny ringing but distortion still a bit off and crackle in the bass at the end.

https://vimeo.com/866797512

That sounds...not good.

You need a fresh set of tubes to rule out the tubes, some of those noises could very well be bad tubes.

Post high res photos of your build.

What cap are you using for the power tube cathode?

Verify your component values are correct to the schematic you used.

Done a hundred highlighter tests but another won't hurt...

Organising alternative power tubes.

It doesn't sound good, but the phone is distorting on the chords and adding to the general ****...

You mean the cap in parallel with the cathode resistors? It's as per schematic I posted a few hundred posts ago, I'll check the actual value. 47uf - 25v printed value on the caps.

Considering getting a used always on UPS to ensure clean AC. Not sure what spec I need, can get this one

https://www.eaton.com/content/dam/eaton ... 201798.pdf

chaliapin wrote: ↑

Fri 09/22/23 2:41 am

You mean the cap in parallel with the cathode resistors? It's as per schematic I posted a few hundred posts ago, I'll check the actual value. 47uf - 25v printed value on the caps.

Oh...that seems not great. The other 36W variants use 100uF: https://mhuss.com/18watt/schematics/36wTMB.gif

Buuuuut I think that's too low. Try 1000uF to 2200uF. I use 2200uF on all of these. 1000uF should be fine too.

I have 100uf to hand but nothing larger, will that be any improvement?

I have 0.2 ohm resistance between ground points on the chassis and very little difference in ac reading hot to ground as hot to negative. Does that cover checking the grounding on the circuit and the mains supply?

Oh and I will mention again that v1 is a 6n2p Russian type of 12ax7. Don't know if that accounts for the low voltage? Maybe bieworm will chip in as I know he uses them.