Trainwreck Rocket AC30-ish build NOW WITH REVERB

PeteC

What happens when you crank it and tap the chassis ? And have you tried tapping the coupling caps with a chopstick when it’s turned well up ? 

Keefy

PeteC said:

What happens when you crank it and tap the chassis ? And have you tried tapping the coupling caps with a chopstick when it’s turned well up ? 

1. Nothing. 2. Both PI grid caps are slightly microphonic in the sense that at tap with a chopstick is audible through the speaker. I have replaced one but I need to order in some spares and replace the other.

Now the big news - THE HUM IS GONE.

Back in this post (nearly a month ago) I tried running the OT primary wires outside the chassis, away from the preamp section of the board. (The constraints of the donor chassis and cab meant that the OT is sited at the far end from the PT, near to the preamp.) At that time it made no difference. I have since re-done the grounding, and today when I tried it - wahey, only the lowest (and perfectly acceptable) level of mains hum remains.

I plugged in a strat and it sounds amazing, even with no reverb. So it was 2 problems, and both needed to be fixed at once for it to work.

To @ICBM and @PeteC - thank you so much for your many helpful comments and contributions on this build, which at times I came very close to giving up on. Also a big shout out to @Modulus_Amps as my first call for the purchase of all things valvey.

However, I don't hear a fat lady singing, and this isn't over yet. My next challenge is to sort the rattle (not currently a problem with the chassis on the bench).

In the longer term, I still fancy adding reverb. My plan is to tap it from the tone stack (before R10) and mix it back in before the PI (after R10). Watch this space...

paulnb57

Great news that the hum is sorted to your satisfaction and 10/10 for perseverance, Id have launched it days ago in a fit of pique!

Keefy

paulnb57 said:

Great news that the hum is sorted to your satisfaction and 10/10 for perseverance, Id have launched it days ago in a fit of pique!

I just had to keep coming back to it every weekend. I wanted it sorted, but I didn't want it dominating my life!

Modulus_Amps

Hi Graeme, Sorry only saw this now that you tagged me in it, I am so busy at the moment that I have not had the same amount of time for the forum.

My layout does not have any circuit current flowing through the chassis, this is crucial for keeping the amp quiet and free of hum. you can't follow half of one grounding scheme and half of another, you have to go full hog otherwise issues can arrise easily.

The phase invertor input in this circuit is a classic place for hum to be introduced and the 220k resistor makes it worse. The  wire layout around the valve socket is critical, you must keep the input wire away from the heaters and as short as possible. (The green wire to V3).  to create space you can raise the wires into the chassis space to create distance between the signal wires and the ot wires and heater wires.

Also keep the wires for V3 away from the wires at V2

The 220k/220k voltage divider is a carry over from Vox/Zwreck designs, adjust to taste and how hot you want the amp to be.

Attaching heater centre tap to the cathode resistor is an easy way to elevate the heaters a little bit.

The way the primary wires are running is far from ideal, if that was an express build it would be squeeling like mad, if they have to run like that you can coil a wire around the length and then ground it at one end to shield it.

Not sure what plate resistor you used on V1, but the TW standard 220k is too high for real people, the amp just does not need that much gain, lowering it to 120k gives slightly more clean range on the volume control and still loads of drive once you get it over 10 o clock with a humbucker guitar.

Shuguang valves sound great, but make sure it is not the source of your hum.

Keefy

Modulus_Amps said:

Hi Graeme, Sorry only saw this now that you tagged me in it, I am so busy at the moment that I have not had the same amount of time for the forum.

...

No worries - I didn't buy a kit, just the components I needed, so it wouldn't have been reasonable to expect build support from you!

My layout does not have any circuit current flowing through the chassis, this is crucial for keeping the amp quiet and free of hum. you can't follow half of one grounding scheme and half of another, you have to go full hog otherwise issues can arise easily.

Yep, learned that the hard way. In fact I've learned a lot from this build. The Valve Wizard stuff on grounding schemes was a revelation. The half-and-half thing came from Rob Robinette's 5E3 grounding scheme, which I naively thought I could just transplant into this amp!

The phase invertor input in this circuit is a classic place for hum to be introduced and the 220k resistor makes it worse. The  wire layout around the valve socket is critical, you must keep the input wire away from the heaters and as short as possible. (The green wire to V3).  to create space you can raise the wires into the chassis space to create distance between the signal wires and the ot wires and heater wires.

Also keep the wires for V3 away from the wires at V2

I'll see what I can improve next time I have the chassis out.

The 220k/220k voltage divider is a carry over from Vox/Zwreck designs, adjust to taste and how hot you want the amp to be.

I may well still tweak that as 470k/220k would be closer to what I need for adding the reverb circuit.

Attaching heater centre tap to the cathode resistor is an easy way to elevate the heaters a little bit.

Unfortunately I don't have that option, as the heaters share their centre tap with the HT secondary inside the donor PT.

The way the primary wires are running is far from ideal, if that was an express build it would be squeeling like mad, if they have to run like that you can coil a wire around the length and then ground it at one end to shield it.

I did wonder about screening them in some way but they now run in a sleeve outside the chassis and pop back in next to the output valves.

Not sure what plate resistor you used on V1, but the TW standard 220k is too high for real people, the amp just does not need that much gain, lowering it to 120k gives slightly more clean range on the volume control and still loads of drive once you get it over 10 o clock with a humbucker guitar.

I used 220k so I might try that.

Shuguang valves sound great, but make sure it is not the source of your hum.

I don't think any of them are to blame in this case.


Thanks for all your comments. My first valve amp build was one of your 5E3 Deluxe kits (which turned out great btw), the second was a 5E3 Micro (approx 0.7W output) that I built into an old Harley Benton Valve Junior and which ended up needing another filter stage to stop it humming. This has been my first grown-up amp scratch build, and I was quite chuffed that it even worked...

PeteC

Great to see that you have it all sorted - I'm sure its an amazing amp now.    And loud .......

Keefy

PeteC said:

Great to see that you have it all sorted - I'm sure its an amazing amp now.    And loud .......

Cheers Pete. Did you say you have a TW-style build in mind or in hand?

ICBM

Excellent . Sometimes with these problems you just have to go back to basics and eliminate all possibilities... especially tricky if there are two contributing factors.

PeteC

Keefy said:

PeteC said:

Great to see that you have it all sorted - I'm sure its an amazing amp now.    And loud .......

Cheers Pete. Did you say you have a TW-style build in mind or in hand?

Yes.  I built a TW Express circuit into an old chassis last month. There is a thread here about it.  Back Street Express build or something like that.  It didn’t get much interest to be honest but it’s a cracking amp   I was v lucky that it fired up with no issues at all first go.  I have since swapped the EL34 to 6V6s to get earlier crunch but it’s still incredibly loud.  I just built a diy Airbrake attenuator for it so I can use it to record in the little home studio.  It’s a beast.  I wanted to build an Express after watching Glen Kuykendalls videos on YT.  Worth a watch.  

Glad you got yours sorted.   I wonder how different it is to an express in sound.  More box and less plexi I guess.  

Keefy

PeteC said:

Keefy said:

PeteC said:

Great to see that you have it all sorted - I'm sure its an amazing amp now.    And loud .......

Cheers Pete. Did you say you have a TW-style build in mind or in hand?

Yes.  I built a TW Express circuit into an old chassis last month. There is a thread here about it.  Back Street Express build or something like that.  It didn’t get much interest to be honest but it’s a cracking amp   I was v lucky that it fired up with no issues at all first go.  I have since swapped the EL34 to 6V6s to get earlier crunch but it’s still incredibly loud.  I just built a diy Airbrake attenuator for it so I can use it to record in the little home studio.  It’s a beast.  I wanted to build an Express after watching Glen Kuykendalls videos on YT.  Worth a watch.  

Glad you got yours sorted.   I wonder how different it is to an express in sound.  More box and less plexi I guess.  

Yes the Rocket is not far off the Top Boost channel of an AC30, although my build has a solid state rectifier. Will look out your build thread.

Keefy

Well I have confirmed that the rattle (more of a jingle really) comes from the valve retainer springs when they are under tension. Even taping them up doesn’t damp them down. They’re probably better for heads rather than combos!

I have found some wire retainers on Karltone so I’ll be ordering a set when their online store re-opens.

Now to plan the reverb circuit...

hywelg

Keefy said:

Well I have confirmed that the rattle (more of a jingle really) comes from the valve retainer springs when they are under tension. Even taping them up doesn’t damp them down. They’re probably better for heads rather than combos!

I have found some wire retainers on Karltone so I’ll be ordering a set when their online store re-opens.

Now to plan the reverb circuit...

Some heatshrink tube over them?

Keefy

hywelg said:

Keefy said:

Well I have confirmed that the rattle (more of a jingle really) comes from the valve retainer springs when they are under tension. Even taping them up doesn’t damp them down. They’re probably better for heads rather than combos!

I have found some wire retainers on Karltone so I’ll be ordering a set when their online store re-opens.

Now to plan the reverb circuit...

Some heatshrink tube over them?

I looked at that. Heatshrink tubing wide enough to slide over the end fittings would not shrink down tight enough on the springs. Also tape wrapped round the springs doesn't work, so why would rubber tubing.

PeteC

wire retainers will do the trick 

Keefy

PeteC said:

wire retainers will do the trick 

Excellent, just waiting for Karltone to re-open!

Keefy

It's been a while since I posted in this thread. I kind of lost interest, and got caught up with a very successful rebuild of a previous project.

I did fit the new valve retainers and they improved matters, but I am still getting some jingling at volume, albeit no worse than in my Matchless Lightning. Once I am all done I will go round the chassis with a hot glue gun and immobilise any potentially rattly wires.

I finally got round to tackling the reverb. I built the driver and recovery circuits (adapted from the donor Carvin circuit) onto tag board which I installed in the space I had left at the back of the chassis, along with the send and return phono sockets. I continued with the Modulus wiring colour scheme when connecting to the valves - blue for plates, green for control grids, yellow for cathodes. While I was at it I re-sited the bypass components for the output valves, thus reducing the length of their cable runs.

Here again are the schematics for the Rocket and for the original Carvin reverb circuit:


Following @ICBM 's advice I
(1) replaced the 12AX7 triode of Carvin V3.B with both triodes of a 12AT7 run in parallel, and upped the power handling of Carvin R25 47k 1W to 2W (I actually used two 100k 1W in parallel as I realised too late that I didn't have a 47k 2W) ; and
(2) replaced Carvin R29 220k with 100k.

The reverb circuit takes off some dry signal before Carvin R18 470k and feeds wet signal back in after it (just before the PI). I have done the same thing with Rocket R10 220k, but upped it to 330k as a compromise (along with another resistor to earth, forming a potential divider not shown in the Rocket schematic).

Here is the completed tag board, and my proof-of-concept lash-up where I didn't want to put a hole in the front panel until I knew it would work:




I fired it up and it works! However, I have to crank the reverb knob up further than I would have expected. I had the driver and recovery circuits powered from B+4 and B+5 respectively, which gave me the following supply voltages:

B+1 335V
B+2 327V
B+3 206V
B+4 159V (driver plate 79V)
B+5 151V (recovery plate 103V)

The plate voltages seem very low, particularly for the reverb driver valve. The Carvin schematic shows B+ values of C=335V (driver) and D=325V (recovery), so I may try taking the reverb supples from Rocket B+2 an B+3.

All the runs of screened cable are grounded at one end only. There is a ground bus on the tag board from which I have run a wire back to the preamp earth. There is also a ground runner to the send phono socket - there needs to be a return path to drive the reverb transducer, and I didn't want to rely on this current running through the chassis. This doesn't seem to have caused any hum problems, but if it does I could look at insulating the socket from the chassis. I also seem to have gone against the colour code convention for reverb send and return, but I can swap the sockets over easily enough.

As ever, any pearls of wisdom gratefully accepted. In the meantime, colour me chuffed!

ICBM

Yes, B+ 2 and 3 would be better choices for the reverb.

But not yet! R24 is far too low a value - the valve will bias much too hot, possibly dangerously so with the increased plate voltage. It’s already pulling a very high current, hence the voltage drop to only 79V. Try 820 ohms.

Keefy

ICBM said:

Yes, B+ 2 and 3 would be better choices for the reverb.

But not yet! R24 is far too low a value - the valve will bias much too hot, possibly dangerously so with the increased plate voltage. It’s already pulling a very high current, hence the voltage drop to only 79V. Try 820 ohms.

Cheers, will do.

Keefy

Some changes made last night and this morning:

• Installed the reverb pot properly.
• Moved the reverb power supply connections to B+2 and B+3.
• Swapped the reverb send/return phono sockets so that the colours correspond to the ones on the reverb tank and match Fender's convention.
• Insulated the reverb send phono socket from the chassis.
• Replaced Carvin R24 220R with 820R.

I now have the following voltages:

B+1 341
B+2 332 (driver plates 176)
B+3 226 (recovery plate 152)
B+4 195
B+5 190

I calculated that the two triodes in the 12AT7 driver are only dissipating 0.54W between them (as against an absolute maximum of 5W) so it's not going to be red-plating any time soon!

There is now plenty of reverb available. There is a noticeable increase in background hum as the reverb level is raised. I will spend some time poking wires with a chopstick to see if I can reduce this with lead dress. At normal playing levels of reverb it is ok though, and to be fair I have a similar issue with my Princeton Reverb 68 Custom.

Here's how it looks at present. The grey wire is a screened cable taking the wet signal to the reverb pot - I have 'flown' it above everything else rather than run it between the valve sockets and under the turret board, and it is higher up than it looks.