Building 1974 style 18W

[BlueX]

Hello, newcomer on this forum. I plan to build a "1974 style" 18W amp (2CH, tremolo, tone controls), but according to modern safety standards and other improvements. Would the Modern Classic in the Downloads section be the best fit?

I plan to source parts myself, and have some related questions:
- Any need for additional fuses, like B+ or heaters?
- Wire rating (AWG)?
- There are two resistors with colour code instead of written values (V3 p1 and p6 to C in the layout). Are these 800k (gray/black/yellow)? I cannot find these in the schematic (instead 220k+100k p1 to C, and 100k p6 to C).
- Any prefered brands, models, or types for the components to try to get close to a 1974 from the 60-ies? I will not source expensive NOS, but am willing to pay for new production with good quality/performance.

A technical question (might be premature, but anyhow):
- Is the preamp ground bus connected to chassis via the pots? I can see that input jack sleeves are connected to ground lug, but I cannot see any direct connection between preamp ground bus and chassis.

I have browsed this forum but cannot find answers to the above questions. I will probably come back with more questions later, and appreciate all help I can get. This would be my second build, after a recent successful completion of a 5E3 kit.

[JMPGuitars]

Welcome to the site!

Yes, the Modern Classic is the best fit for what you describe. the threads in my signature for more general helpful information.

No need for additional fuses. Some people like to add a fuse to the power transformer, you can find that in some schematics if that interests you.

Those colorful resistors are 100K Carbon Comp. That's the only place CC resistors might be beneficial, but you're welcome to use whatever you like. I generally prefer CMF series 1W metal film for the regular resistors in these amps.

My favorite transformers are Heyboer from GDS (gdsamps.com). I like the Mercury Magnetics as well, but they cost twice as much, and I dunno if anybody can hear a difference.

New production Mullard, TungSol, and Sovtek tubes are good (I prefer Mullard). Do NOT use JJ tubes, they're trash.

The preamp ground bus is the grey line on the turret board in the layout. Do NOT put a bus bar on the pots, that's a huge opportunity for noise issues and ground loops. All preamp grounds (including the pots) go through the bus on the board, and all of it gets grounded at the input jack ground (follow the bus wire to black wire to input jack ground lug to chassis ground via ground lug). In a two-channel amp like you're going to build, this gets done at the higher-gain channel. In a TMB amp, that's the TMB. I don't know if it matters in the standard classic.

The most important thing you can do is watch the soldering videos, and practice a bit more before building while waiting for your parts. If you look at the Tremolo TMB series, you'll see I have a BOM in there. Many of the parts there (https://www.mouser.com/ProjectManager/P ... 0eb19df5c8) can be used for the classic. Just pay attention to what you actually need for your build.

Thanks,
Josh

[BlueX]

Thanks for a fast and comprehensive reply!

I will spend some time now to check components from different suppliers. That BoM link is useful! I'm located in Sweden (northern Europe), and e.g. Digi-Key offers free shipping to Europe for orders above USD 40. However, with smaller companies shipping can be really expensive. Need to check alternatives.

I noticed your signature when I browsed this site. Very useful info!

Now I see the connection between board ground bus and chassis, via input jack, to the far right. When you're looking hard for something it can sometimes hide in plain sight.

All preamp grounds (including the pots) go through the bus on the board, and all of it gets grounded at the input jack ground (follow the bus wire to black wire to input jack ground lug to chassis ground via ground lug).

[JMPGuitars]

Thanks for a fast and comprehensive reply!

I will spend some time now to check components from different suppliers. That BoM link is useful! I'm located in Sweden (northern Europe), and e.g. Digi-Key offers free shipping to Europe for orders above USD 40. However, with smaller companies shipping can be really expensive. Need to check alternatives.

Use this site for Mouser: https://www.mouser.se/

[BlueX]

Some more questions:
- Does the Modern Classic circuit work without footswitch connected? TubeTown has a 150 pF cap across the footswitch jack in their kit to avoid oscillation in case of no footswitch connected.
- Any experience with parts from Modulus Amplification? Their transformers are made by Dagnall to replicate vintage parts, as I understand.

I looked for Heyboer transformers. Couldn't find any here in Europe, and shipping from the US was about USD 75.

[Bieworm]

Some more questions:
- Does the Modern Classic circuit work without footswitch connected? TubeTown has a 150 pF cap across the footswitch jack in their kit to avoid oscillation in case of no footswitch connected.
- Any experience with parts from Modulus Amplification? Their transformers are made by Dagnall to replicate vintage parts, as I understand.

I looked for Heyboer transformers. Couldn't find any here in Europe, and shipping from the US was about USD 75.

You don’t need that cap on the switch jack. I build those all the time with an isolated switching cliff jack. Just connect the ground lug of the cliff jack to ground and connect both switching lugs.

There are other options for transformers in Europe. The inmadout transformers from tube town are great. Those are the hammerpaint finish ones. They are a little high in voltage for the B+ though (300-0-300 secondary)
Another great and very cheap power transformer is the Primary Windings PWAM01. Those are my go-to for 18 watts.
For OT I use hammond 1750 series

[BlueX]

Primary Windings PWAM01

Thanks!
Look's like the PWAM01 has only one 6,3 V secondary. Do you use the same heater circuit for all valves (pre- and power, and rectifier)?

[Bieworm]

Primary Windings PWAM01

Thanks!
Look's like the PWAM01 has only one 6,3 V secondary. Do you use the same heater circuit for all valves (pre- and power, and rectifier)?

There is plenty of heater current on tap to feed the preamp, power amp and rectifier. I personally don’t do tube rectifiers in 18 watts. There’s no real benefit from an EZ81 and it will give up sooner than any SS rectifier

[BlueX]

...and connect both switching lugs. (footswitch jack)

One more question (I'm still not completely into all the amp lingo):
Does this mean to connect the tip and sleeve lugs that disconnects when putting in the plug?

Thanks for all support!

[Bieworm]

...and connect both switching lugs. (footswitch jack)

One more question (I'm still not completely into all the amp lingo):
Does this mean to connect the tip and sleeve lugs that disconnects when putting in the plug?

Thanks for all support!

Yes. The ones that don’t move

[BlueX]

Where can I find A2M pot?

In the Modern Classic layout and schematics it says "A2M" for the tremolo speed pot. I'm unable to find this. B2M (linear) is available, but the highest value for A (logarithmic) I can find is 1M.

I have checked with Modulus Amplification, Tube Amp Doctor, Tube Town, Mouser, Digi-Key, and Distrelec. Any other place to look that has these?

[Bieworm]

Where can I find A2M pot?

In the Modern Classic layout and schematics it says "A2M" for the tremolo speed pot. I'm unable to find this. B2M (linear) is available, but the highest value for A (logarithmic) I can find is 1M.

I have checked with Modulus Amplification, Tube Amp Doctor, Tube Town, Mouser, Digi-Key, and Distrelec. Any other place to look that has these?

I use the Fender spec C3M.. it’s perfect

[BlueX]

I use the Fender spec C3M.. it’s perfect

Thanks!
Now that I looked for C3M I noticed that Modulus actually has the A2M (could be new of course).

[JMPGuitars]

I use B2M. These: https://www.amplifiedparts.com/products ... 38-bushing

C3M is reverse taper. I suppose any of those would be fine, just be aware of how they operate.

[BlueX]

More questions on the Modern Classic layout and schematic:
- Is the 2A Slow Blow fuse for 110-120V operation?
- Any recommendations for 230-240V? 1A Slow Blow, or something else?

[JMPGuitars]

More questions on the Modern Classic layout and schematic:
- Is the 2A Slow Blow fuse for 110-120V operation?
- Any recommendations for 230-240V? 1A Slow Blow

Yup.

[BlueX]

Is there any chassis template (like drill template) for Modern Classic as a head? Main question is where to put the 32+32 uF cap can.

By the way, is there any search function for the forum? I can only find "Search this topic".

[Bieworm]

Is there any chassis template (like drill template) for Modern Classic as a head? Main question is where to put the 32+32 uF cap can.

By the way, is there any search function for the forum? I can only find "Search this topic".

You can’t search outside the topic when inside the topic. But on the home page you can..or when you click forums

[JMPGuitars]

Bieworm is correct. You can click the breadcrumbs above to go to Forum and search there, or under quick links, it will take you to a broader search page.

Thanks,
Josh

[BlueX]

Finally got almost all parts. My next step will be to dry-fit to check possible layout, and what else I need (like fasteners, etc.)

Most parts are from Modulus Amplification (UK), some from TAD (DE), and Mouser.

Parts.jpg