1987/2204 50W Combo Build

[mnegrete]

Greetings. After building my son a couple of amps based on fender circuits, he's decided he likes the Marshall sound. I tried to convince him he needed a Plexi/1987 circuit, but after listening to a bunch of clips online he came to the conclusion he wanted a 2204 circuit. After researching a bit on here I came across the combo build that SDM developed a layout for and Neikeel further modified, so I think I'm going to go for the best of both worlds and build a comb.

I'm still in the planning phases, but wanted to get input on a few things:

1) As he isn't targeting an exact sound, I'm thinking of going with Classic Tone transformers (40-18089 laydown PT, 40-18025 OT, and the 40-18032 5 Hy choke). I've read great things regarding Marstan and ****** transformers, but it seems like the classic tone transformers may give the best bang for the buck. Any thoughts on whether the Classic Tones are up to the task?

2) Looking at the 2 in 1 layouts from SDM's site, it looks like SDM and Neikeel go about the controls in two different manners. I've seen a third option where one of the channel two inputs is dropped and a 1 Meg pot is placed in one of the input jack holes and a DPDT switch is placed in another input jack hole. Is it really just up to personal preference on which one of these layouts is used to switch between the circuits?

3) I'm going to be building a 2x12 cab and am looking at filling it with a pair of Celestion GT12-75 speakers. I've also considered using Vintage 30s, and the third option is using one of each. Any recommendations?

Thanks,

Matt

[danman]

Classictone iron will work great! All of the brands that you mentioned are excellent and I wouldn't have a problem using any of them in a Marshall style circuit. I used the Classictone stuff in my last two builds and have been very pleased with the amps. Just make sure to choose the correct secondary voltage PT as they do offer a couple different versions. The late 70's 2204's were running a lower B+ of around 370-380vdc and many folks say these were some of the warmest and thickest sounding amps. I built one last year using the 40-18095 with multiple secondaries. The 560v ct delivers around 370vdc at the plates while the 625v ct comes in around 412vdc in my amp.

[mnegrete]

Danman, thx for the input.

I'm still waiting for the transformers to be delivered, but have wired up the board while I wait for delivery.

I've been looking at the wiring for the push pull pot that switches between the tie circuits and I'm wondering if there is anything in particular I should be concerned with regarding the routing of the wiring for the switch. There is quite a lot happening in a small place, and want to make sure the wiring is as tidy as possible.

[neikeel]

I used shielded wire on the grid wires and tuck the 0.0022uF cap up in the corner of the chassis under the pots (use shrink wrap on the lugs!).

If you use stock values (or maybe 8k2 on the cold stage cathode - I like best) it is very quiet and effective.

Steve did an alternative layout (not as easy to mod and original back to stock) which is quieter if you plan on modding with bypass caps on the cold stage or really lowering the R down to 4k7 or sinilar.

I like 8k2 on the cold cathode, unbypassed but on the treble peaker (usually 470pF) with the 470k between stages I found 1000pF or 1500pF works well for more grit and gain. IIRC I used 1800pF tubular LEMCO on the last one. This has loads of gain (it is a 100w with stock early 70s Dagnalls) partcularly good at pinched harmonics and down tuned stuff.

[mnegrete]

So, the transformers finally arrived yesterday and I'm probably 75% complete with getting the board wired in. A question or three on grounding:

I have the preamp Gain, MV, and Mid pots all grounded together, along with the rest of the pre-amp wires.
The center taps (high voltage secondary & heater) from the power transformer are grounded to the chassis at the Screens/PI filter.

Should the grounds for the bias and presence be separate wires back to the ground connection at the Screens/PI filter?
Should the ground wire for the preamp bus be connected to the ground connection at the Screens/PI filter?

[neikeel]

[mnegrete]

So, last night I finished the wiring. Prior to putting the tubes in I was getting 328v coming off the secondary of the PT. B+ measured 456v at both cap cans and also at the OT primaries.

After putting the tubes in and turning it on, while in standby mode, the tubes glow. When I flip the standby switch on the mains fuse blows.

I've been over the board, the power circuit wiring, and the tube sockets and nothing is jumping out at me as being wrong. Any thoughts on where to start troubleshooting?

Thanks!

[danman]

I am guessing from what you said that the fuse does not blow without the power tubes installed but does blow when you power up with the tubes. Do you have another set of power tubes to try? You may have a bad tube in the set. Before trying the new tubes though, take a bias voltage reading at pin 5 on each power tube socket. Make sure that you have around -40vdc at each socket. Posting some good clear pics of everything will also help with the troubleshooting.

[danman]

I forgot to ask...is it the mains fuse or the 500ma fuse for the B+?

[mnegrete]

I forgot to ask...is it the mains fuse or the 500ma fuse for the B+?

It's the mains fuse. I realized I put in a 2A fuse, but it looks like I may want a 3A fuse with 120v AC input to the PT. I would think that without pushing the amp that a 2A would suffice, but maybe that's the problem?

Also, you are correct that the fuse does not blow when the tubes are not installed, but then does blow when they are installed.

The tubes are new and I don't have another set to swap them with. I'll measure the bias voltages and report back.

Here are photos:

[danman]

Yes, the amp requires a 3amp slow blow fuse for use with 120vac input. Check for proper negative bias voltage first and then see if the correct 3amp fuse holds with the set of power tubes that you have. Really nice looking build you did! I did notice a piece of clipped component lead laying on the turret board....did you get that out before powering up?

[mnegrete]

Really nice looking build you did! I did notice a piece of clipped component lead laying on the turret board....did you get that out before powering up?

Thanks. Between the instruction manual that George put together for the Metroamp kits, and the layouts that Neil and SDM have put together, it makes keeping everything clean pretty straight forward. However, the input jacks and push/pull pot didn't end up as clean as I had hoped. Also, the OT wires were a bit unruly.

Good eye on the lead fragment floating around. I blew the amp out and tried to shake all the loose bits out before powering up, but will double check that it's not still there.

[herbvis]

I just had this same issue with ht fuses blowing. It was a bad tube for me.

[mnegrete]

Got it figured. I replaced the 2A fuse with a 3A fuse. That kept the mains fuse from blowing, but then the power tubes started red plating. The 500mAh fuse blew, and then after some more trouble shooting I realized that I had forgotten to install the unshielded wire between the Dual pot and board as part of the PPIMV.

Once I installed that, everything is looking good and sounding good, too.

Danman, thanks for the hint on measuring the voltage at pin 5 on V4 and V5. A zero voltage reading there led me to the problem.

[danman]

Glad you got it straightened out!