COS build

[Auke Jolman]

Hi All,

First of all I wish you all the best for this new year. Hope you and your loved ones are ok.

After not being able to get the former COS-build hum-free, I decided to do a complete rebuild of it. I changed the layout quit a bit. The amp is now completely free of hum. As with my first COS I used Tubenit's schematic (see attached). The layout I made is also attached.

The thing is that on the clean channel I can't dial in een clean tone. Even with the volume on 2-ish, it's already heavily distorted. I've been over it a couple of times, but can't seem to be able to find why that is the case.

I checked the voltages on the different nodes and following tube pins:

Node A: 366
Node B: 354
Node C: 321
Node D: 289
Node E: 288
Node F: 305
Node G: 303

        1      6        7      8
V1  125   131
V2  239   146
V3                    93    116
V4  288   131
V5  225   217

V6-3: 360
V7-3: 242

A picture of the interior of the amp is also attached.

A little help in resolving this issue is appreciated.

With regards,

Auke

[Glenn]

Hi Auke: I dont have the program to look at the schematic but if V6 an V7 are the power tubes, you seem to have a problem with V7
if these are your plate voltages..
V6-3: 360
V7-3: 242 - this is way low and might be why you have only distortion.

sorry only a guess on my part

[sluckey]

V7-3: 242 - this is way low and might be why you have only distortion.

I agree.

Swap V6 and V7. Does the V7 socket pin 3 still read low?

[Auke Jolman]

Thanks for the reply.

I checked my notes. V7-3 is 342 (not 242). Still a difference with between the 2 powertubes.

I don't have another pair of 5881 at the moment. I will try a pair of 6L6. I will also  hook up the pre-amp via the FX-loop to another amp and see/hear how that sounds. I'll keep you posted.

[Auke Jolman]

Hi All,

Today I made some progress in a way. I discovered that I had miswired the secundairy leads of the OT. The Common was wired to the 16 ohm tap and vice versa. I fixed that, but now when I turn the amp on, it's motorboating after a few seconds, at least that what I think it is. There's a blue glow in the output tubes that's intermittent and there's a thumping sound coming out of the speakers. That was probably also the reason why the readings on Pin 3 of the outputtubes where of.

What is the course of action(s) I can/must take to fix this problem(s)

[PRR]

If swapping the secondary fixed one problem and caused another, flip the primary also?

First disconnect the NFB. If that makes it loud/rude but stable, then flip.

[Auke Jolman]

While looking to disconnect the NFB I spotted a wiring mistake at the presence pot. I changed the layout accordingly (I had connected the 390R resistor at the wrong side of the 0,1 uF cap).

- Disconnected the wire from the presence pot to PI: amp sounded a bit darker, no thumps, no squeels;
- After connecting the wire from the presence pot to PI, I disconnected the wire from the speaker out jack: amp made a very faint and distorted sound.

Not sure what to do next. I did not swap the secundary leads of the OT.

I did nog get that intermittent sound as I described in the previous post. The output tubes I used did give a lot of blue glow, more so when the amp was turned up. I had a pair of 6V6's in another amp and put those in this amp. No blue glow or thumping sounds. The amp sounds fine to me. So maybe I blew up the other output tubes?

So I put them in another amp. They seemed to function fine in that amp. Turned the amp way up to maybe stress them a bit more? No blue glow whatsoever.

This makes it more confusing to me. What to do next?

[tubenit]

I am not clear exactly what you did with disconnecting the presence pot? 

Q1) Did you ground that point on the LTPI when you disconnected it?  IF not, try that and report back.

Q2) Can you confirm that with the 6V6's you tried that everything on the amp worked fine to your satisfaction and there
was no hum?   IF that is the case, I'm thinking the previous output tubes may be the issue even though you stated they
worked OK in another amp?

Some tubes give a blue glow that is normal for that tube.  My concern is if a tube is "red plating".

With respect, Tubenit

[Auke Jolman]

@Tubenit:

Q1: I disconnected the wire going from the presencepot and wire clipped it to ground at the "ground-side" of the cap.

Q2: The 6V6's I took out off another amp worked fine. I played it for about 1 hour, clean channel and overdrive channel. The amp has no hum whatsoever. No redplating and no blue glow when driven hard. The other tubes (a pair of 5881, 6LC and 6V6) gave intermittent blue glow/flashes and with each flash (every second or so) a low thump that came through the speaker.

I've put the 5881 in another amp and played it for about 15 minutes. No problems that I noticed.

[tubenit]

Thanks for the clarity by answering those questions.

I am suspecting those 5881 tubes were having some oscillation and/or redplating in the COS?  It may be that in the other
amp the tubes are not driven as hard by the preamp tubes or that amp is biased differently allowing the tubes to work OK
in the other (non-COS) amp?

I think I'd be inclined to try a different pair of 5881 tubes in the COS and see if those work well like the 6V6 do?

Overall, without the hum …………. how do you like the tone of the amp?  What kind of speaker are you using?

with respect, Tubenit

[Auke Jolman]

@Tubenit:

I ordered a new pair of 5881 tubes today.

Changing the layout around especially relocating the relayboards made a huge difference in maintenance. The amp is completely hum free. The hiss level is maybe a bit high. I think I'll replace the 33K grid resistors with 10k resistors in conjunction with e ceramic cap to ground and see if the hiss level drops a bit. Once I start playing is no problem though.

Using the 6V6's gives the amp a little darker color. I think I like the 5881 better in there. I like the amp a lot. The clean channel is very full sounding. I have to find the sweetspot for me, but I'll get there when I've got the new pair of 5881 in there. I've put in a trim pot in the back to make the overdrive in the clean channel a bit less I used a 100K pot followed by a 56K resistor adding up to the 150K resistor preceding the third gainstage. Since I've put it in, I've not dialed it back, so not sure wheater I'll keep it in.

In the overdrive section I've changed the bypass cap on the first gain stage from 5 uF to 0,68 uF. I found the overdrive was een bit dark sounding. Did that just 1 hour ago, so I'll have to play with it some more. I want to play with some backing tracks and see if the amp cut's through the mix.

It took some time to finally get the amp up and running but it is well worth it. When I get the new 5881 in there I'll try and make some soundsamples.

The wiring error on the OT was also present in the former COS amp I build, so that may have contributed to nog being able to solve the problems with that amp.

I'm using an amp top amp box so I can crank the amp to my linking and choose the speakers I like the most. At the moment I'm using Celestion Greenbacks.

[PRR]

....Not sure what to do next. I did not swap the secundary leads of the OT.  ...

Am I confused? You just said you did swap the secondary.

....I had miswired the secundairy leads of the OT. The Common was wired to the 16 ohm tap and vice versa. I fixed that...

[tubenit]

I'll try and make some soundsamples.

Please do!  I'd love to hear them!  Thanks!

I prefer 5881's more than 6V6's in mine also. 

I am using a Emminence Delta Pro 12A with mine.  I like this the best with the clean channel of any speakers I've tried.

I also tried a Warehouse ET65 with the COS.  To be honest, the OD sounded the best to me with the Warehouse speaker.
The OD was super smooth with it. However, I felt like I lost a little with the clean channel using that speaker.

with respect, Tubenit

[Auke Jolman]

@PRR:

You are right. I did swap the secondary leads. I ment to say the primary leads

[Auke Jolman]

Hi All,

With a new set of 5881 the amp is free of problems

I guess wiring the OT the wrong way doesn't make tubes last very long

Made a few changes:
- Replaced the V1 33K grid resistor to 10K with a 120 pF to ground. This reduces the hiss level significantly;
- Put in a Frondelli master volume;

For the moment I disconnected the 100 pF cap on the volume pot.

I'm very pleased with the amp sofar. I will post back with some soundbits.

Thank you for all the comments and help.

[tubenit]

Look forward to your sound bits very much!  THANKS for sharing them! 

When it's all done, would you be willing to share a new schematic of your COS, please?

With respect, Tubenit

[Auke Jolman]

The schematic and layout are included as attachments.