Quad Preamp Issues

[Wolfster]

I recently scored an old quad pre (from '88, if the date on the reverb tank is anything to go by), and I gave it almost immediately to my amp tech to take a look at it. He noticed that one of the LDR's was bad, so he replaced it, and I also got him to convert it to 240V (I'm in the UK). I only just got it back after over a month's wait, and I've noticed a few things which are up with it:

1. Noise is pretty bad, especially on Lead 1. This seems weird to me, since this is the lower gain lead channel, but the noise is roughly twice as loud as that of Lead 2. I've tried it with a couple of different valves, and the problem is persistent.

2. EQ's on both channels suck a noticeable amount of gain. Also, if I engage the EQ of one channel with the footswitch while playing on the other, it still has this effect.

3. There is a sort of "boinging" sound, a bit like plucking a tight rubber band, when palm muting on the lower strings using Lead 2.

My amp tech is honest and scrupulous, and knows his stuff, but he's been very busy recently and might have overlooked something, especially since the quad is a complicated beast. I can't really ask him about these issues since he basically said that he never wants to see the thing again (!); apparently some of the circuit seemed quite mysterious to him and he spent lots of time trying to understand it, with limited success.

Any help with clearing up these issues would be extremely gratefully received, even if it's just suggestions for other techs (in the UK, preferable close to SW London), or for other forums where I might find help. I really thought that this would be the heart of my dream rig, but so far it has been mostly just a source of frustration and disappointment. I know that there are, or have been, people on here who have a lot of experience with the guts of the quad, so hopefully you'll be able to help an electronics noob like me! If sound clips would help, let me know and I'll post 'em up. Thanks a lot in advance!

 

[crane]

Hi !

1) What type of noise ? Hum, hiss, plops ? Have you tried every valve position ? signal in lead1 goes through V21A -> V21B -> V23A and B -> V12B -> V24A and B for reverb -> V14A and B -> V22A or B for output A or B. So the signal goes through 6 tubes. If the problem is only on lead1 search for V12, V21 or V23 first.
Lead 1 is no low gain channel at all, it's based on the MkIIC+, remember Metallica ? ;-)

2) This seems to be normal, it depends on how you tweak the EQ. If you set it flat there should be no volume differences between "with" or "without".

3) The Quad is tricky to tweak, try differents settings, remove reverb, lower the bass, etc...

 

[Wolfster]

Hi Crane,

Thanks a lot for your response. Here are mine:

1. The noise is just hiss, but there's lots of it. I know that Lead 1 is high gain, but as I understand it, Lead 2 is higher gain, and yet it has far less hiss; this is what made me think that there's something wrong. In any case, I'll try messing around with the valves more and see if it makes a difference.

2. The difference which I'm talking about is not to do with volume, but the level of saturation of the distorted tone, which engaging the EQ decreaseses noticeably, even if set flat. Setting the EQ higher boosts the volume as it should, but still leads to the loss of saturation. Also, the EQ on channel one, say, affects the tone of channel two. Could this be a problem with LDR's?

3. I'd tried some tweaking before posting yesterday, and the "boinging" sound seems to be fairly persistent. I'll try some more and see what happens.

Thanks again!

 

[daveyboogie]

Hey Wolfy...

Yep, I've been there alright.

Had similar probs with my Quad when I imported one but basically, all my issues only cleared up when I changed ALL the LDRs associated with channel switching. These LDRs can do strange things including cause noise and you might have an issue with volume fading out and in. I replaced about 10 of them but when I checked the old ones on the bench I couldn't find a fault in any of them! yet all my probs had gone away and it's been very reliable ever since.

Not much fun stripping down a Quad but it's worth doing. Just change everything you can afford while it's in bits. Also remember to change ALL your plate resistors for higher rated ones: I had probs with these things too. It's a long, hard battle learning to live with a Quad but hopefully you'll find it's been worth it in the end!

RS is cheapest place for LDRs.

HTH.

Dave.

 

[crane]

Ok I agree with daveyboogie. I had a lot of problems looking like yours with my Quad when I bought it. I've changed every LDR. I'm a confirmed tech (I don't know everything about tube amps, but using a solder iron is part of my job ^^), so no problem, but first, LDR is'nt costless and tech job neither ;-). I live in Swtizerland and I order LDR from Germany (lower price I could find) and it cost me about 160$ for 24 LDR. Only for stuff, job was mine.
That's why I want to focus your problem, but with the Quad configuration it could be tricky and surely be faster to change all LDR (as I've done, I'don't wanted to spend 3 years on it).
But with all new LDR, a small volume change still occurs when using both EQ at the same time.

If you don't want to change all the LDR we have to check further for your problems.

Like Daveyboogie said, increasing anode resistors will surely increase gain, but it's not like the Quad was made for, and you'll lose some dynamic. And it will change valve polarisation too. Problem is elsewhwere, Quad has gain for sure.

 

[Wolfster]

Daveyboogie,

I've read a lot of your posts on the forum from some time ago, and I know that you've done a lot of work to your Quad, so I was hoping that you'd respond! Thanks a lot for your input. As I understand it, you recapped your quad, ie replaced the big blue things (sorry, but as I said I know next to nothing about electronics), but you've said in a previous post that you don't think that it helped all that much, and that the LDR's were the most important thing. Can I ask what else you changed, and how to tell which LDR's needed changing and what are their specs etc? My tech said he couldnt find any tolerance information on which LDRs were needed.
Also, what problems were you having with the plate resistors and why did you change them for higher rated ones? Lastly, can I ask you whether you think that the quad's worth the trouble which you've had with yours? I have to say that I'm thinking that it might just be better to cut my losses and pass it on, so a bit of inspiration would be great before I spend any more time and money on the thing!

Crane,

Thanks for your interesting response. I really don't have money to burn, and I've already spent quite a bit on this recent work, so I'm probably limited to doing the labour myself. Do you think that this is realistic for someone like me, who has limited experience with a soldering iron, but is willing to take time and care? With the EQ problem, as I said, it's not volume that I'm losing so much as saturation/gain. The fact that they affect the other channel makes me think that perhaps some of the signal is bleeding into the other channel's circuit when I'm switching them in, but I really don't understand how this can be happening, since I thought that they come after the gain stages.


Thanks a lot to both of you once again.

PS. Regarding the other problems, I haven't had time to try tweaking anything since my first post, but I'll do so this weekend and let you know what I find.

 

[crane]

The LDRs are quite easy to change, you only have to respect polarisation (+ and -). But you have to remove the board from the chassis. This could be done by unsoldering all the pots, the EQ, etc... lot of job but not complicated. Be sure to take pictures and to notice "what goes where" if you decide to do it by yourself.
As I can remember there are 20 VTL5C1 and 4 VTL5C4 (Vactrol brand).
The lack of gain is strange. I don't know what settings you use but with volume and lead drive at 10 and a good V shape EQ you should have something heavy.
Maybe a combination faulty LDRs could partially derivate the signal and decrease gain.

 

[daveyboogie]

Wolf...

I think Crane has said everything you need to know about the LDRs. I didn't change all 20-odd of them, just the 10 or so associated with channel switching and isolation. If you're having probs with the EQs too then changing those LDRs would definitely be worth a go.

Like I said in the earlier threads; I changed the big blue caps purely because I had the Quad in bits and thought it might be worth doing. Not really sure it improved much in the way of performance but at least I don't need to worry about them now. Incidentally, I tested the old caps and found them to be in near perfect condition.

The reason I changed the plate resistors was that I found some of them had been stressed and were showing much lower values than they were supposed to be. One was completely blown effectively shutting off Lead-1. All the plate resistors showed some sign of stress- It was like someone had taken a bad tube and tried it in all the different tube positions one after the other. I changed them for identical values to those spec'd in the schematic but just used slightly larger watt values. Low gain probs in your lead channels could be down to something like this.

Can't really advise whether you'll get on with on your Quad after you've thrown a load of money at it... I've had mine about a year and I'm only now starting to get some really serious tones out of it. I personally think it's a journey worth taking but you have to be prepared to throw everything you know about tone out of the window and start again from scratch.

Here's everything you need to know about Quad LDRs etc...

http://tubefreak.com/s-quad3.gif

 

[crane]

The reason I changed the plate resistors was that I found some of them had been stressed and were showing much lower values than they were supposed to be. One was completely blown effectively shutting off Lead-1. All the plate resistors showed some sign of stress- It was like someone had taken a bad tube and tried it in all the different tube positions one after the other. I changed them for identical values to those spec'd in the schematic but just used slightly larger watt values. Low gain probs in your lead channels could be down to something like this.

Very interresting, I'll check mine !

Can't really advise whether you'll get on with on your Quad after you've thrown a load of money at it... I've had mine about a year and I'm only now starting to get some really serious tones out of it. I personally think it's a journey worth taking but you have to be prepared to throw everything you know about tone out of the window and start again from scratch.

Yep, I agree, the Quad is very difficult to tweak and seems to react very differently than any other amp. You have to forget how to tweak an amp and start from zero using your ears, exactly like daveyboogie said.

 

[Wolfster]

Hi guys,

I messed around with the valves yesterday, and while their placement changed the tone, the hiss on Lead 1 was still present. Also, I noticed that engaging the EQ on Lead 2 not only sucks gain but also volume, which suggests even more strongly that there's a LDR issue. So I opened up the old girl, and I counted 19 optoisolators in total, which agrees with the list you sent me, Daveyboogie. At first I was confused by the numbering system (seems to jump from 15 to 21) but then I realised that this was because there are 15 VTL5C1's ("type 1") and 4 VTL5C4's ("type 2"). I've had a look online and it looks like I can get a complete set for around £75, which seems reasonable (http://uk.farnell.com/eg-g-vactec/vtl5c1/optocoupler-single-channel/dp/1652517 and http://uk.farnell.com/eg-g-vactec/vtl5c4/optocoupler-single-channel/dp/1652520?crosssellid=1652520&crosssell=true&in_merch=true&).

So at the moment, I'm considering replacing all the optoisolators and the plate resistors, so I'd like to ask you both a bit more about the latter. Are they just normal resistors? How do I know which ones they are? How can I check whether they've been stressed or not? What power tolerances would you recommend getting?

Regarding the procedure itself, I know that I should drain the capacitors, ie the big blue things, but is there anything else that I need to do before I can work on it safely?

And last but not least, what are all the orange things in there?

Thanks a lot!

 

[crane]

Orange things are tone capacitors, named orange drops, used for the tone stack (bass mid treb....) and also used as filter caps between gain stages.

I can't remember what size are the plate resistors, but you can use metal oxide resistors, 2W 5%. Just check their size and space available for installing them.

For draining the big capacitors use a 220Kohm or more 2W resitor (220'000 ohm at least, big value, I insist otherwise the cap will be drained too fast with a big "bang"), solder one side to a clip and the other side to something you can easily manipulate (multimeter probe, ...). Be sure that everything is well isolated (thermal tube for example). Unplug your Quad (or any other amp). Clip your home-made device to Quad's ground and then touch each + of big blue capacitor for 20-30 sec. If you add a led after the resistor (minus to ground clip, + to resistor) you can see when there's no more tension in the cap.

 

[daveyboogie]

learn this drawing! you can clearly see the plate resistors and values.

http://tubefreak.com/s-main.gif

just put you ohm meter on the ht side of the big blue cap [after discharging it] and measure the resistance on pin 1 or 6 of the tube socket [depending on which stage you're looking at]. If the values are more than about 5% out of tolerance then I would change them -

...infact, just change them anyway.

oh, and you'll need this one too.

http://tubefreak.com/s-lead.gif

...and possibly this one:

http://tubefreak.com/s-main2.gif

 

[Wolfster]

OK guys,

Here are the schematics with what I believe to be the plate resistors circled; I'd really appreciate it it you could let me know whether I've got it right:

I note that V14A appears not to have plate resistor, and I don't really understand what's going on at V24B; will the 22K resistor above it need replacing too? Sorry for the stupid questions, but as I said I really know next to nothing about how these devices function.

Incidentally, at the same time I'm going to try and make the modification described in the following thread (second to last post), to even out the volume differences between the two channels:

http://forum.grailtone.com/viewtopic.php?p=125881&highlight=#125881

Daveyboogie, I've noticed in your sig that you use a Peavey Classic 50/50 with your Quad, as I do. I was wondering if you've tried it out with other poweramps, and how they've compared to the Peavey?

Cheers guys!

 

[daveyboogie]

Wolfman...

For now, i would just concentrate on replacing the plate resistors associated with Channel-1 / 2 and Lead-1/ 2. Check the values of the other resistors [in the reverb and EQ sections] compared with the schematic and see if they're anywhere near the given values. Just replace anything that looks dodgy. Don't worry about V24b etc... Just get the issues sorted.

I've thought about changing my Peavey to another poweramp for a quite a while but still not really sure what to do yet. Was wondering if the smoother sound of a 6L6 might help things for a bit more clarity and definition.
I retubed the Peavey last year for JJs and I now use it on half power [just using 4 tubes] because I find the 50/50 blisteringly loud! It's taken about a year of getting used to but I'm now hearing glimpses of some very brutal tone from this thing. Make no mistake, that's a very serious pre you've got there!

 

[Wolfster]

Cheers for the advice Davey, I've ordered the parts and I plan to do the work this weekend; I'll let you know how it goes.

 

[Wolfster]

Hi guys,

So I said that I would report back once I'd done the work, so here goes:

I'm sorry to say that it went very badly, as far as results go. In the end, I just replaced the optocouplers, and now the only channel which is at all usable is Lead 2. When switching through the channels, the front panel LED's don't come on individually, but most of them come on to some degree. As far as I can see, there are two likely explanations. The first is that my solder joints are bad. This was my first assumption, so I went through and resoldered all of the joints. I should mention that some of the pads came away from the board, which may also be the cause of the problem. The other possiblity is that I damaged some or all of the optocouplers. I've tried testing some of them with a nine-volt battery connected across the LED legs, and measuring the resistance of the LDR. The from panel LED's come on, but the measured resistance doesn't change. Is the the right way to go about this?

So all in all, my quad is is a pretty bad way, and I'm really not sure what to do next. Any advice would be extremely gratefully received.

 

[daveyboogie]

oh well, it was definitely worth a try....

Looks like you'll be dropping it off with me, then!

 

[Wolfster]

Hey DaveyBoogie,

Where abouts in the UK are you based?

 

[daveyboogie]

I'm up in North Yorkshire...

How often do you gig up this way? It's either that, or post the Quad.

 

[[Beast]]

Hey Mesa/Boogie fans, greetings from Slovenia.

I've been a Quad owner for 4 years and I just love this amp. But... I have a problem with it, quite similar to the ones, described in this thread.

It all started, when I purchased Nobels MS-8 midi swithcer (before that I used Rockman midi octopus). The Nobels I bought online had a mind of its own and froze quite often and is now history. But... whilst using it with Quad, the midi switcher used to switch the EQ2 on Quad on and off or rather the LED was going on and wouldn't turn off, but there was no change of sound.

After I quit using Nobels, I noticed a change in the Quad's sound, especially the first channel. The first thing was, I have to push the gain to 10 to get distorted sound, the sound is really thin and annoying, pinch harmonics are almost impossible to pull of, the thing is really dead. The second thing are these volume drops, that accur. Sometimes it happens on rhythm 1 (clean), but most of the time it happens on lead 1. The sound just goes away, all you can hear from the cabinet is a buzzy, thin little sound. The sound comes back, if I turn the channel volume knob. Also when switching the channels, the lead 1 is just dead.

Also, there is no chugga chugga sound, when palm muting, especially on the channel 2.

Please tell me, could this be a LDR issue or is it a tube issue? The tubes were all changed last year (JJs).

Tnx for your answers and help, I really need this pre, 'cause our band is playing a lot and I really don't have an adequate back up. I was thinking about buying a Carvin Quad X pre and save the money I'm going to spend on Rocktron Patchmate8 Loop, but I just love Quad's sound too much.