I thought I had this figured out...

Littlewyan · Post by **Littlewyan** » Fri Jul 24, 2020 9:51 pm

martin manning wrote: ↑Fri Jul 24, 2020 6:14 pm
Littlewyan wrote: ↑Fri Jul 24, 2020 3:18 pmJust a quick note, I recall when I used this circuit in one of my amps that I had to be careful with the standby switch. I can't remember the exact issue, but a paragraph on Merlin's site explains it well:

Notice that this charging path includes the HT load represented by R3. This means that before the valves have warmed up, there will be no bias voltage present unless you have a bleeder resistor across the HT.
Littlewyan makes a good point. Are you measuring with standby set to play, and if not, is there a bleeder resistor across the reservoir cap?

Can you draw out the original low voltage and bias circuit?

And do you have any valves in the amp when testing this circuit? If not, you’ll definitely need a bleeder resistor.

sluckey · Post by **sluckey** » Fri Jul 24, 2020 10:38 pm

Littlewyan wrote: ↑Fri Jul 24, 2020 9:33 pm Spot on. Although dorrisant is getting very weird readings. I don’t think they should be reading any dc between the cap and diode.

His meter is averaging the mostly positive pulses at the junction of the diode and cap. It is an ac waveform but the zero volt baseline is shifted way down such that the waveform is mostly positive with only a small negative portion. Even my Fluke 87 thinks it's dc if the meter is set to dc mode. Look at the scope pics in the link I posted. Should be apparent.

martin manning · Post by **martin manning** » Sat Jul 25, 2020 1:01 pm

dorrisant wrote: ↑Sat Jun 27, 2020 5:48 amOn the other end of the class xy cap (@ the reversed diode) I measure 87vac and 53vdc. After the reversed diode there is less than 1 vac and -7 vdc.

AC and DC after the cap look fine, but it still seems you have a very low DC resistance to ground after the diode. What do you measure?

dorrisant · Post by **dorrisant** » Mon Jul 27, 2020 11:52 pm

Late as usual... here's some pics.

This is an old hand drilled board I used because it was in the drawer and I've used this Merlin ckt before with good results.

Keep in mind, the 10k from pot to ground was removed so it could be probed with a decade box.

dorrisant · Post by **dorrisant** » Mon Jul 27, 2020 11:56 pm

Phone is giving me fits...

Now on the computer. Can't post these two pics. WTF?!?!

Tried to post like 20 times. keep getting this:
"ERROR
It was not possible to determine the dimensions of the image. Please verify that the URL you entered is correct."

Whatever, this site has been doing this to me quite a bit lately. Getting a bit sick of it.

They are not too big... 2.5 MB and 2.8 MB.

Some things are just cursed. This is how it has been with every turn with this shit amp.

Post by **pompeiisneaks** » Tue Jul 28, 2020 12:25 am

That almost sounds like it's trying to link to an external image instead of a locally uploaded one. If you use the 'image' tag but it's a webpage it will give that kind of error. It has to be an actual image link.

.jpg or the like.

~Phil

dorrisant · Post by **dorrisant** » Tue Jul 28, 2020 12:31 am

Understood. But two pictures, no problem. Two others are giving this error. All four taken at the same time with the same device. All four were uploaded at the same time. Only two work. Emailed all four to myself. Still no dice on the two. The same two that were already posted will upload without issue... just to test theory/troubleshoot. Renamed them - no go. Tried to PM them to you. That ain't working either. Email address?

dorrisant · Post by **dorrisant** » Tue Jul 28, 2020 1:39 am

Preamp tubes are in, no power tubes.

All plate voltages look about right for said conditions.

Less words... More pictures.

What do you know... this one works!

Post by **pompeiisneaks** » Tue Jul 28, 2020 1:47 am

Can you send them to me via email? I'll see if I can figure it out.

~Phil

dorrisant · Post by **dorrisant** » Tue Jul 28, 2020 1:50 am

pompeiisneaks wrote: ↑Tue Jul 28, 2020 1:47 am Can you send them to me via email? I'll see if I can figure it out.

~Phil

Thanks Phil.

Sent.

Post by **pompeiisneaks** » Tue Jul 28, 2020 2:36 am

So I've figured it out. There's a bug in this version of phpbb for jpg files if they have mismatched exif data for the image dimensions... basically, there's no easy fix, unless you save as a non jpg format. Or edit the exif data and fix or remove the size data.

/sigh.

I tried converting to png but the files are now too big.

I'll have to try resizing them as well.

~Phil

Post by **pompeiisneaks** » Tue Jul 28, 2020 3:12 am

there you go.

~Phil

Littlewyan · Post by **Littlewyan** » Tue Jul 28, 2020 9:10 am

1. Are you testing this circuit with the standby switch on?

2. What's the purpose of R3 and D1?

3. Do you actually mean -1VDC at output?

4. Is that solder built up on the negative end of C1? What measurement do you get if you measure resistance to ground from that point?

martin manning · Post by **martin manning** » Tue Jul 28, 2020 11:21 am

Littlewyan wrote: ↑Tue Jul 28, 2020 9:10 am2. What's the purpose of R3 and D1?

See here: http://www.valvewizard.co.uk/bias.html second schematic from the top of the page.

Littlewyan · Post by **Littlewyan** » Tue Jul 28, 2020 11:23 am

Ah thanks Martin. Can't remember if I implemented that on mine....

Tube Amp Archive

I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...

Re: I thought I had this figured out...