New 183 build

[Stephen1966]

Interesting questions I did ask though are why not just use coax for the master? And why not?

If using coax you are killing a lot of heighs.. if not, it is ok.
explanation, worst case:
Master volume 1M on low positions , makes cca 900k resistor in series with the mid viper, which is loaded to coax and has capacitance at least 50pF, you have rollof too early, starting after 1kHz. Plus PI input, long tail pair Miller capacitance.

Master should be at cca 250k to avoid this effect, and loaded with some cathode follower to not to load too much previous stage.. I have already tried output from the CF, 100k and 250k Master volume it is working better than 1Mohm

Thanks, I see that now... No coax is being used on the master though, and I have the CF FX loop inbetween the master and PI. The FX loop, designed from the Dumbleator, certainly affects the tone (in a good way) but I will have to see how it sounds here before I go trimming the value of the master pot.

Looking again at the lead dress in the 183, I'm pretty confident on this amp, MrD chose to route the blue master wire, from the relay (COM 2) to the master (pin 3) close to the OD coax, under the boards. Possibly to avoid the relays' B+ and ground wires tucked into the edge of the chassis. The red wire from the master wiper and the white wires between the presence pot and cap (on the board) all seem to follow a similar route. Some elements of the lead dress are more critical than others. When I fire this baby up, we may see that's the case here.

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[Stephen1966]

A bit more progress today... I'd get a lot more done if I didn't have to work a full day's work as well. Not complaining though; as far as problems go, it's nice one to have.

Today then, the bias circuit went in along with the cathodes for the power tubes and the 50/100W switch. Some other stuff, you can see here as well. The layers are going in nicely now. Tomorrow, I'll be mounting the OT and choke.

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[Stephen1966]

Today, the OT and choke went on.

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The Tube Town OT is a bit different in that the primary and the secondary wires emerge from one side of the transformer rather than secondaries from one side and primaries from the other. MrD's 183 transformer was the latter type and his secondaries enter the chassis somewhere under the PI circuit and then pass between the spacers for the preamp and PS boards. To do this, I had to add spacers (washers) under the feet of the OT to provide about an 1/8" space underneath to route the secondaries to the opposite side of the transformer. The feet and the core of transformer, otherwise, being flush. Because of the weight of the transformer the feet (mounting lugs) are susceptible to deformation. If you look at the 183 photos you will see the transformer has done this there - probably caused just by being placed down hard a couple of times. To mitigate this I used extra wide diameter washers so hopefully, this will be a bit more robust.

In this photo, you can just make out the gap I created under the transformer...

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Internally, I decided to use a heavier gauge, higher voltage rated wire for the screens, and the blue primary for the OT is left intentionally long in case the phase needs to be reversed. I noticed the 183's plate primaries had been swapped - I won't be soldering the NFB wire in place until I'm sure I have the correct phase.

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[Stephen1966]

Some more progress over the weekend...

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The last time I installed an FX loop, I used all shielded cable, which works fine, but is very time-consuming. I've kept it to a minimum this time and there's a slightly more rational layout.

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[bepone]

Nice, first time to see this combination of resistors in one amp, PRP, Holco + other modern MF, couldn't wait for the results

[Stephen1966]

I'm looking forward to firing the thing up as well, so I've been getting a move on. It's starting to look like an amp now

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I'm hoping to get the power transformer and the other bits and pieces around it, installed next. And that should be it. It should be up and running by the end of this week.

[Stephen1966]

Most of the internals ready for a start up now, just need to wire up the filaments. Tested the GND connections thoroughly and checked for any shorts - looks good.

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[sluckey]

Very nice workmanship!

[jazzbass]

Hi Stephen1966,

I follow what you write on TAG of your #183 from the first post and I congratulate you for the accuracy of the construction. I hope you post a demo in the discussion soon.
I have a question for you, what model and where do you find the trimmers you insert into the amplifier, the ones I use have adjustment with a screwdriver and are uncomfortable.

Thanks, Franco

[Stephen1966]

Cheers Steve and Franco! Couldn't have done it without everyone's help.

Franco, if you mean the Pihers for PI and OD adjustment, they are the PT-15 series: https://cz.mouser.com/ProductDetail/Amp ... SIeQ%3D%3D. Be aware, you need to buy the thumb-wheels separately: https://cz.mouser.com/ProductDetail/Amp ... umQ%3D%3D . With thumb-wheels are the type I prefer and are easily available in the values we use, though if you check the datasheet, you will see there are other configurations available.

If you mean the trimmers on the FET board, they are Bourns' pots: https://cz.mouser.com/ProductDetail/Bou ... qg%3D%3D . The 3926Y series.

In the meantime, I've got the filaments wired up. Solid core wire, nice and rigid, unlikely to oscillate.

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Taking a break now. Tomorrow's the big day.

[jazzbass]

Hi Stephen,

Thanks for the info

Franco

[bepone]

I saw on the weather map one big cluod is coming from Czech Republic. Is the amp alive??

[Stephen1966]

I saw on the weather map one big cluod is coming from Czech Republic. Is the amp alive??

No smoke nor fire here It lives!

Power up went smoothly but the voltages were a bit wonky. That was expected as some of the values in the dropping string were a guess.

The TF-130 had HT voltages at 340-0-340 I believe, and the tube-town transformer has HT voltages at 350-0-350. I was thinking I might have to drop the B+ with a zener diode but in the end, it wasn't needed. I had the PT primaries wired up for 230VAC, the European "standard", but that was hot and my filaments measured something like 6.7VAC so first job was to rewire the 230V tap with a 240V tap, and then set the Variac for 230V. That gave me a B+ of 467VDC originally, with a 6.1VAC filament voltage. So with that settled, I started work on the dropping string...

Here is what the new PS board looks like now:

DELTA 183 - PS board(2).jpg

The biasing will be done tomorrow now, for the time being, the bias voltage is set to max. negative voltage.

With the new values in the dropping string though I have a pretty good set of voltages across the tubes now.

On the PS board, we have:

B+ OT 469.3V
B+2 SCR 468.4V
B+3 PI 418.2V
B+4 FX 363.8V
B+5 FX 256.3V
B+6 OD 319.3V
B+7 CL 311.6V

PI
V4a
(pin 6) 295.2V
V4b
(pin 1) 290.3V
V4 tail 55.6V

A little cooler than the original but there is a nice 5V difference between the plates. This is with the PI trimmer set to midnight. So probably, very close.


FX
V3a
(pin 6) 256V
V3b
(pin1) 224.8V
V3 tail 29.6V

These are all close enough to the original Dumbleator's 260V, 230V, and 30V respectively so again, probably good.


OD
V2a
(pin 6) 200.5V
(pin 8 ) 1.75V
V2b
(pin 1) 200.5V
(pin 3) 1.7V

All within 5% of the original. A different tube might make a difference here but this is good enough to be getting along with.


CL
V1a
(pin 6) 196V
(pin 8 ) 1.7V
V1b
(pin 1) 200.5V
(pin 3) 1.62V.

This is pretty close and a near perfect match with V2. It looks like V1 is a little hotter. I will try swapping V1 and V2 and see if that gets me closer to the original. The final 2k2 resistor in the dropping string should take care of slightly lower voltages in V1.


Without the bias being done, the plates and screens of the power tubes are not worth mentioning yet. I also need to check I have the correct polarity of the OT with the NFB but that will be easier tomorrow when I hook up a speaker. There is some fluctuation with the mains supply here, so some of the measurements drift (up and down) between a first measurement and subsequent measurement. Annoying! But I'm kinda used to it.

At least with the dropping string sorted, that's most of the unknowns dealt with.

Not celebrating just yet - I've got to hear it yet - but it's looking promising

[Stephen1966]

It lives and breathes!

Swapped V1 and V2 - this is a better balance.

V2
(pin 1) 203.3
(pin 6) 199.4
(pin 3) 1.62
(pin 8 ) 1.73

V1
(pin 1) 195.7
(pin 6) 194.4
(pin 3) 1.63
(pin 8 ) 1.67

Tested the NFB and OT and polarity is okay, only couldn't resist plugging a guitar in so I didn't connect up the NFB tonight.

Biased it up to 20mA and gave it a go. First impressions...

Very quiet and very loud. Grounding appears good with an excellent, low noise floor. With the master up halfway and volume at about 15% it's plenty loud enough. NFB will tame it a bit. Even with such a cool bias though, the OD is bordering on raucus if a bit anaemic, and in high bright settings - which isn't its natural territory, anyway - there is a hint of unpleasant distortion. It was at this point though, I was starting to get bit carried away so decided to call it a day. Connecting up the NFB and biasing it properly will make a world of difference, I'm sure. The OD isn't particularly transparent/articulate yet, but I'm sure with a little fine tuning I can get a cleaner delineation between the strings, it's just a bit muddy at the mo. Perhaps, a different cathode cap there will be the solution. OD trimmer is just at 50% though and that needs to be advanced to give 20k-30k to ground I think, in line with the conventional wisdom of these things. PI trimmer is also set simply at 50% - a 10k trimmer with 100k on one side and 110k on the other, giving a 4.3V difference between either side - still, one of the nice things that is clear even with these rough settings, is that it wants to sustain like the devil. High notes are clean, as you would expect, and full, even with the cool bias. There is some touch sensitivity but I don't have a particularly clear view of that at the moment. It's all a bit academic when it isn't properly tuned up or voiced yet, but I have vague impressions it is there already. All in all, it goes well. No real problems so far. This is fun.

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[ijedouglas]

Congrats! Looking great!