18W Plexi clone multiple issues

[Waldlander]

Hi to all amp and solder lovers ;-)

I have built an 18 Watt Marshall Clone with a Kit (tube-town.net) It was my 2nd tube amp and I have built approx. a dozen stompboxes. (PCB-kits and vero boards). The boxed and the previous amp (3 Watts, kit from musikding.de) all work very well. Overall I’m considering myself still as fairly new with DIY electronics and don’t have any background in this field.
Now I’m facing some issues with the 18 Watt. It works, there is an amplification, but certainly not in the way it should be. Here is the behaviour of the amp:

Normal Channel
- no signal at all on low channel
- amplification on high channel, but relatively low. When volume pot is turned up fully, the amplification is slightly above listening level and still clean without distortion

Tremolo Channel
- on both inputs much more amplification than on Normal Channel
- With fully turned up volume pot on high channel and a humbucker guitar (fully turned up) only medium distortion (and not very well sounding - somewhat tinny)
- no tremolo effect at all, also no change when tremolo intensity pot is turned - except that the signal gets slightly louder when turned up

Overall
- All tubes are glowing
- The signal sounds clean and there is no noise or hum - but it’s certainly not the desired amplification level, distortion etc.

Measurements
- The transformer looks good. the values around V1 are almost as desired. But the values around V2 and V3 are deviating considerably (attached Scans: measured values in green)

One more point: When switched to standby mode there is still sound to be heard - but very weak. (I have no experience with Standby mode, but I don’t assume that this is normal, or is it?)

I have checked the wiring several times and can’t find any mistake. Of course I am happy to provide gut shots if this helps the help ;-)

My main questions are:
- Where and how can I start to trouble shoot?
- Could this be a single part/ failure in the circuit or does this sound like there are several issues?

Many thanks for any help in advance!
Sebastian

[pdf64]

One triode of the LTP phase splitter doesn't look to be conducting.
Try a known good tube, check all components, values, wiring, solder joints for the LTP.
Try taking some detailed photos of your build; upload them to a hosting site and post the links, so that others may review where the problems may be.

[Waldlander]

Thank you for the quick reply and this hint. I've checked already the tubes (replaced them with tubes from another amp), they seem to be working. But I will check all the other points you mentioned.

Cheers
Sebastian

[big_teee]

A voltage chart of tube pins, can be helpful.
Here's an instruction manual with testing steps in section 8.
It might shed some light of what is missing.
https://d1sjrnpi226dnf.cloudfront.ne...pdf?1382030428
T

[loudthud]

Looks like pins 3 and 8 should be connected together on each preamp tube socket. Verify those connections.

[Waldlander]

Looks like pins 3 and 8 should be connected together on each preamp tube socket. Verify those connections.

Yes, that was exactly the point! Even after checking several times, I just overlooked these connections.
Thank you very much! The amp works now fine and sounds really nice :-)

Cheers!

[Steve A.]

Glad to hear it works now! I had a question: how do you like the tone control? Does it give you effective control over the tone (from a bit too dark to a bit too bright)? Just wondering how well it works compared to a Fender tweed tone control...

Thanks!

Steve Ahola

[Waldlander]

Glad to hear it works now! I had a question: how do you like the tone control? Does it give you effective control over the tone (from a bit too dark to a bit too bright)? Just wondering how well it works compared to a Fender tweed tone control...

Thanks!

Steve Ahola

I haven’t played that much since the amp is just working since yesterday. But the tone control so far seems very effective.
I just checked again on the high channel. From around 3 to 5 o’clock it seems even to cut some low frequencies and from around 7 to 9 o’clock it really cuts some highs (almost like a filter). In-between it’s nice for “flavouring” according to taste. (Tried out a humbucker bridge position, tone pot on guitar fully bright and then dialed the highs back on the tone control, around 11 o’clock and it gave me a very sharp and present sound with some body :-)

[Steve A.]

I haven’t played that much since the amp is just working since yesterday. But the tone control so far seems very effective.
I just checked again on the high channel. From around 3 to 5 o’clock it seems even to cut some low frequencies and from around 7 to 9 o’clock it really cuts some highs (almost like a filter). In-between it’s nice for “flavouring” according to taste. (Tried out a humbucker bridge position, tone pot on guitar fully bright and then dialed the highs back on the tone control, around 11 o’clock and it gave me a very sharp and present sound with some body :-)

Thanks! The 18 watter tone control might be a nice upgrade to the standard Fender tweed tone control. (On a Fender amp I might want to reverse the pot terminals so that 10 is brighter. Would need to use a reverse audio pot which are easier to find these days.)

Steve Ahola

[Richard]

Looks like pins 3 and 8 should be connected together on each preamp tube socket. Verify those connections.

I love these troubleshoot threads, it's like watching House.