Metropoulos Forum

I'll send you a PM. Just need to sort out the last few resistors. I have decided to go with a GDS power transformer for now, and see if an original pops up.

BTW, if anyone else is looking to build an 18w, then you too can build one with an original OT for a reasonable (or at least I thought it was) price:
http://www.ebay.co.uk/itm/RADIOSPARES-R ... SwuAVWwetw

BTW, found a 2.7M on Ebay, so I got those last little resistors sorted. Still need the 1k5 and 125R, but hopefully I can get those from Valvestorm, although RS ones would still be very tempting.

One question for those in the know; the 100R screen resistor looks to be a 1W carbon comp on the originals. I would think that's a rather low wattage, OTOH it's only 100R and thus wouldn't drop much voltage, and I don't know the current draw of EL84 screens. I have a NOS carbon comp that looks just right.

shakti wrote:BTW, found a 2.7M on Ebay, so I got those last little resistors sorted. Still need the 1k5 and 125R, but hopefully I can get those from Valvestorm, although RS ones would still be very tempting.

One question for those in the know; the 100R screen resistor looks to be a 1W carbon comp on the originals. I would think that's a rather low wattage, OTOH it's only 100R and thus wouldn't drop much voltage, and I don't know the current draw of EL84 screens. I have a NOS carbon comp that looks just right.

I cracked and used a 100R 2w Piher 9same size).

let me rummage through the RS box tonight...............
Edit:
5w RS in 100R, 150R, 1k, 1k2 and 2k.

Getting very close now! I am trying to make this one look "vintage" and have tried to keep a little more "slop" in there than I usually do, but a few tweaks have found their way in, such as using shielded wire on V2 per Neil's recommendation.

And we have ignition!

Unfortunately, I got the classic phase reversal howl on first fire-up. I swapped the primaries over, but to my dismay I got the same howl. Any ideas?

The secondary of the OT is not grounded. I couldn't see that the originals had any ground connection on the secondary. OTOH, I would think it needs to be grounded just like we ground the common connection on a self-lead OT via the speaker jack. Do any of you 18w builders know?

Good news; I reversed the secondaries this time, so I had the ground on the output cable (sleeve on the output jack, running into an attenuator) connected to the black wire terminal on the OT secondary (I believe this is the common on the OT). No more howl!

I am still waiting for one 68k input resistor for the trem channel, so that one is not working properly yet. The normal channel, however, works exactly as it should with no squeeling, and glorious, touch-sensitive tone! There is a tiny amount of ground hum at extreme volume settings, but not too bad. Nevertheless, more hum than I am used to in my other builds where I use Larry grounding. I stuck with the standard grounding scheme for this amp, but grounded the heater CT on the ground buss on the back of the pots. I did not use shielded wire for the normal channel inputs.

This was just a very quick preliminary test through an attenuator into a 4x12. Now on to mounting the amp in the combo cab and hooking it up to the two alnicos...stay tuned!

shakti wrote:Good news; I reversed the secondaries this time, so I had the ground on the output cable (sleeve on the output jack, running into an attenuator) connected to the black wire terminal on the OT secondary (I believe this is the common on the OT). No more howl!

I am still waiting for one 68k input resistor for the trem channel, so that one is not working properly yet. The normal channel, however, works exactly as it should with no squeeling, and glorious, touch-sensitive tone! There is a tiny amount of ground hum at extreme volume settings, but not too bad. Nevertheless, more hum than I am used to in my other builds where I use Larry grounding. I stuck with the standard grounding scheme for this amp, but grounded the heater CT on the ground buss on the back of the pots. I did not use shielded wire for the normal channel inputs.

This was just a very quick preliminary test through an attenuator into a 4x12. Now on to mounting the amp in the combo cab and hooking it up to the two alnicos...stay tuned!

Well, after mounting in the combo cab, rewiring to 8 ohm and connecting the two 16 ohm alnicos in parallell, I am getting another issue. There is a terrible buzz which goes away when I connect a guitar. And when playing, I get an intermittent bad crackling noise. So I am guessing that a ground connection has become bad. No time now to do troubleshooting, but hope to be able to go over it sometime during Easter.

I don't know...doing this as a semi-restoration project and trying to stick to stock layout and details, has given me a new appreciation of the clone/"tribute" amps I've been doing before. Using an improved grounding scheme a la Larry certainly has some great advantages. And whoever came up with the idea of a ground buss on the back of the pots should have a permanent place in the Hall of Shame. It's such a bad idea in so many ways I don't even know where to begin...

The stock grounding on these does work assuming all connections are clean and good with tight nuts on 'cutting' ground lugs,

My 18w is pretty quiet. Getting the trem channel stable was the only issue I had.

No time yet to work on it, but I did notice one thing on your build Neil. I am certain it is not the cause of my current noise, as it was not there before I wired it into the combo cab. But I have the transformer screen and HV CT grounded along with the mains filter cap. Heater CT is grounded on the buss bar. On your build, Neil, you have the screen (violet wire) grounded to the buss bar, and the HV CT and heater CT both grounded along with the mains filter cap.

I don't really know what the transformer screen does and where it is best to ground it, but I chose to wire the heater CT closer to the input as that is what I usually do per Larry's recommendation.

Update:

I don't think this is a grounding issue as such. First of all, I moved the mains earth to a separate lug right next to the inlet. I had it mounted to the rectifier tube socket, and it was actually a touch loose, but should have made good enough contact.

I any case, there was still a loud buzz present with the amp switched on and nothing connected. I isolated it to V1, as it goes away when pulling V1 (or V2), but not when pulling V3. Plugging in a guitar makes the noise go away. However, I discovered that the noise is only there with the tone control for that channel turned all the way down. Turn it up just a tiny bit, and it goes away. I believe this is a known issue with these amps, but it's usually described more as a squeel than a buzz, and I'm not sure if plugging in a guitar is supposed to solve the problem. I tried to swap V1 and V3, but that was even worse, as I got a very loud wailing oscillation with that particular tube in V1.
So, playing through the normal (non-trem) channel with the tone turned up a bit, it sounds beautiful, clear and touch sensitive, but as soon as I turn it up a little bit, I get a crackling distortion that comes in wheneve I hit a little harder. It's frustrating, because I can tell there's a really *great* sounding amp in there! It seems to mate very well with the alnicos, BTW.

A few things I am concerned with; I have used a lot of ceramic caps in here, and I actually don't know their voltage rating. The 10k Lemcos appear very, very similar to the RS ceramic caps that were used in many original 18-watters, so I am sort of guessing those are good. The 5k caps I used were a mixed bag, they all measured good for capacitance value, but I don't know the voltage rating. Could these be symptoms of failing caps?

And another small detail; I discovered there's a hairline crack in the shield base on V1. The socket base itself appears to be fine though. I might replace it just to be sure...