Sundown A 50 - mod ideas?

[Darkbluemurder]

I recently acquired a used Sundown A 50. These amps were built by former Ampeg member Dennis Kager. While it is a practical and compact combo the clean sound is too bright for me (anything above 3 on the treble knob is plain shrill) and the OD sound is too dark. Furthermore, the Celestion G12-S50 sounds rather flat and boxy to me (I compared it to the G12-100T in my Deluxe Reverb II and a D-style 1x12 cab w/ Jensen Neo).

Now my questions:

1. Any speaker recommendations for a full strong sound with good bass, articulate midrange and smooth highs? I play Country Rock and use clean, crunch and overdrive sounds so I tend to an allround speaker.

2. I have some ideas for mods to make the amp less boomy and a bit brighter in the OD channel. Nevertheless if somebody could share experience I would very much appreciate it.

Thanks a lot and keep on rockin'

[oldhousescott]

For a speaker, you might try an Eminence Wizard. Reputed to be a good all-around speaker according to a Tonequest report (FWIW) hitting on the points you mention. I like Weber's Ceramic Signature 12B which is very similar to the discontinued Reverend Alltone 1250 which Kager and Joe Naylor spec'd for the Hellhound amp (the drive channel on the A50 is very similar to the Hellhound's single channel which itself is very similar to the Marshall 2203/2204).

For clean channel mods, you could remove or reduce the bright cap on the clean channel. 250pF is a tad strong, you could try something between 50 and 100pF. Also, I pumped the values from the schematic for the bax stack into Duncan's TSC.....ewwww. If you have the TSC, I would suggest the following mod values:
R1=100k
R3=10k
C1=470pF
C2=4.7nF
R4=180k
C3=150pF (or 120pF)
C4=1.5nF (or 1nF)

For the drive channel, I would remove or reduce the cap paralleled to the MV. I would change the gain boost cap from .1uF to .68uF. You could also parallel a .68uF on the 820 ohm cathode resistor. If it's still muddy, you might have to fiddle with the reverb send. The 470pF in series with the 5nF is rolling off some highs to ground. You could put 220k in series with the 470pF to reduce the treble rolloff. This will also reduce the strength of the signal going to the reverb however, which may not be a bad thing.

Hope this helps.

[Darkbluemurder]

Thank you very much for the tips. I also thought about reducing the cap in parallel with the OD Level control.

What struck me in this design was the 220uF cathode cap in the first gain stage - I believe 22uF would be more than enough for a full range response together with the 2k7 cathode resistor. In fact my D-style modded Concert was a bit muddy in the low end while it had a 22uF on the paralleled stages of V1 (together with a 1k5 cathode resistor). I replaced it with a 10uF and the muddiness was gone.

I am aware that the original tweed Bassman and JTM 45 amps also used a 220uF on the first gain stage but these amps have a lot less gain than the A 50.

Any opinions?

[rfgordon]

Finding the right value for something like a coupling cap can be a frustrating, time-consuming search. I'm still thanking the guy who suggested that I make myself a multi-cap tool for just such occasions. (Works great for resistors, too.)

Get yourself a small project box and a rotary switch. Install a bunch of caps of typical values for whatever you intend to work on (you could have one box for coupling caps and another for K resistor bypass caps, or even expand to one for Mallories vs ODs, etc.). Have the in and out soldered to clilps. I mounted mine on wood strips so they can sit atop an open chassis without touching anything.

Use the shorting type of rotary switch so you aren't letting a pulse of DC thru every time you turn the switch.

You may find that this test apparatus is a bit noisy, since it effectively lengthens your grid wire a bit while it's in place.

I think Weber even sells these things already made up now, so they must be getting fairly popular.

Cheers,
Gordo

[Darkbluemurder]

For a speaker, you might try an Eminence Wizard. Reputed to be a good all-around speaker according to a Tonequest report (FWIW) hitting on the points you mention. I like Weber's Ceramic Signature 12B which is very similar to the discontinued Reverend Alltone 1250 which Kager and Joe Naylor spec'd for the Hellhound amp (the drive channel on the A50 is very similar to the Hellhound's single channel which itself is very similar to the Marshall 2203/2204).

Thanks for the info, especially on the Emi Wizard. However I also read many comments on that speaker re. cone cry. I have never consciously heard a speaker cone "crying". I also heard that this is a phenomenon which appears only when playing real loud so I am confused as to whether this is an issue. Any alternatives in the Eminence Red Coat line which sound similar but do not have the cone cry (Red Ryder, Tonespotter)?

[Bob-I]

Dennis Kager.

If it helps, Dennis now works at Central Jersey amp repair, which is in the same building as Lou Rose music in Edison NJ. He knows that amp inside and out.

[Darkbluemurder]

OK, today I took a shot at the OD channel. Here is what I ended up with for now:

1. Removed 1000pf to ground after the tone stack. This took away the dull sound.
2. Changed treble cap from 1000pf ceramic to 500pf silver mica. This made the sound a bit less nasal.
3. Changed bright cap on gain pot from 1000pf ceramic to 500pf silver mica. To me this makes the sound a bit more open and less midrangey.
4. Changed that freakin' Ck of the 1st gain stage from 220uF to EDIT 1uF. This took care of the boominess. This affects the clean channel, too.
5. As a compensation for the lost bass from step 4 in the clean channel (the first gain stage is common to both channels) I upped the 0.022uF PI coupling caps to 0.1uF.
6. Changed V4, which is for the 3rd OD gain stage and the cathode follower, from a 7025 to a 12AT7 (yes, that's right, a 12AT7). This step was done to prevent the 3rd stage from going into cutoff.
7. Changed Rk of V4b from 100k to 22k. This is to prevent the cathode follower from going into cutoff. Steps 6 and 7 together made the amp feel more dynamic and less harsh. For more information on steps 6 and 7 check John Atchley's site (www.guitarnuts.com). He did the same things to his Pignose G-40 which is basically the OD channel of the Sundown (also designed by Dennis Kager).

Well, the OD is much better now. It sings better, the tone controls are much more effective, and different guitars can be discerned. Before the mods all guitars sounded the same through this amp. Still there is room for improvement. All wires are shielded. I have not yet measured the capacitance but the wire diameter is very small which indicates high capacitance. I also had to do some basic improvements such as twisting the OT primary wires and adding bleeder resistors (I believe it is plain irresponsible to not include them in an amp).

That's it for today. I will take a shot at the clean channel once I got the new supply of caps. I will probably try the values suggested by oldhousescott. I put them into the Duncan Tonestack calculator and they seem to provide a lot more bass - which would be exactly what I need.

Thank you all for your input.
Cheers Stephan

[skyboltone]

(I compared it to the G12-100T in my Deluxe Reverb II and a D-style 1x12 cab w/ Jensen Neo)

Stephen, share some thoughts on the Jensen Neo. I'm pretty resistant to trendy stuff; alnico works just fine thanks, but this magnet sounds interesting and not the usual manufacturers "cheaper is better" response to the market.

How do you like it and compared to what.

Dan

[rfgordon]

The guy I built the jazz amp for had purchased a 15" Jensen neo speaker. It turned out to be the perfect match for the amp. Very warm and clear without being woofy or boomy. The highs are good but not as sharp with a 12 or 10, but that works out very nicely for jazz. Put in a pine cab it makes for an easy carry, compared to some of the big magnets out there!

I may check out some of the other neo speakers like the Celestions. If only there was a neo speaker with a hempcone.......

[Darkbluemurder]

(I compared it to the G12-100T in my Deluxe Reverb II and a D-style 1x12 cab w/ Jensen Neo)

Stephen, share some thoughts on the Jensen Neo. I'm pretty resistant to trendy stuff; alnico works just fine thanks, but this magnet sounds interesting and not the usual manufacturers "cheaper is better" response to the market.

How do you like it and compared to what.

Dan

I compared the Jensen Neo to a Weber Ceramic Blue Dog in my Deluxe Reverb II. Initially the amp came with a EVM 12" which I replaced with a Celestion Greenback. I later replaced the Greenback with the Weber. I thought the Weber was far better than the Greenback. When I put in the Jensen Neo I did not notice any degradation compared to the Weber. The Celestion G 12 100 sounded fatter than the Neo with more lows and a little less highs. Unfortunately I have not yet compared any of the Eminence models to what I have.

Today I made 3 more changes to the Sundown's drive channel:

1. Reduced coupling cap from the 2nd OD stage from 0.022uF to 0.0047uF to further reduce flubbiness.
2. Changed Ck of V3b (2nd OD gain stage) from 0.1uF to 0.47uF (really wanted 1uF but didn't have one) to make the boost mode sound fatter.
3. Added an 1 Meg resistor between the treble wiper and the master volume to reduce harshness.

Although the amp sounds better than what it started off it appears to have its limits what can be done. I replaced the PI tube and V3 (the first tube in the OD section) with JJ ECC83S but not much improvement. The power tubes are Sovtek WXT6L6. Anyway I will try to change some more caps and see what I can do.

[Darkbluemurder]

OK here we are with the final evolution of the Sundown A-50:

I first checked the bias and found it way low: 21ma at a plate voltage of 400V! I set it to 36ma which is still a bit low but that was all I could do without changing the bias circuit. But even that helped a lot in eliminating the crossover distortion.

1st gain stage (clean and OD): changed 220uF Ck to 1uF.

Clean channel: changed bright cap from 270pF to 100pF - DONE.

OD channel: changed 1M resistor before gain pot to 390k. Very important! The larger resistor kills highs when turning down the guitar volume pot!

Changed Rk of the 2nd gain stage from 10k to 4k (added an 8k2 in parallel) and the boost cap from 0.1uF to 1uF.

As a compensation for the increased gain I inserted a 470k resistor right after the 0.0047 coupling cap (there was no resistor there before).

Finally I added a 100pF snubber cap between the PI plates and reduced the feedback resistor from 100k to 56k.

Now the amp sounds like a guitar amp! No need to call it "Sundown" anymore - "Sunrise" would certainly fit it better!

Cheers Stephan

[jbefumo]

I have my Sundown SD1012C in a combo cabinet driving an Altec 15" - with a frequency response up to 8000Hz, no issues with bright!

I NEVER used the overdrive channel - just too over the top for my needs, but if you look at the schematic, if you eliminate that first preamp stage, what you have left is pretty much a Plexi Marshall. I rewired mine that way, revoicing the 'green' channel a bit to resemble a Dumble OD special clean channel, and the red channel to pretty much stock Marshall specs -- in other words, I basically have two clean channels. With the switchable post PI master (Governor), and judicious use of an external attenuator, I have as much OD as I want.

Joe

[quote="Darkbluemurder"]I recently acquired a used Sundown A 50. These amps were built by former Ampeg member Dennis Kager. While it is a practical and compact combo the clean sound is too bright for me (anything above 3 on the treble knob is plain shrill) and the OD sound is too dark. Furthermore, the Celestion G12-S50 sounds rather flat and boxy to me (I compared it to the G12-100T in my Deluxe Reverb II and a D-style 1x12 cab w/ Jensen Neo).

Now my questions:

1. Any speaker recommendations for a full strong sound with good bass, articulate midrange and smooth highs? I play Country Rock and use clean, crunch and overdrive sounds so I tend to an allround speaker.

2. I have some ideas for mods to make the amp less boomy and a bit brighter in the OD channel. Nevertheless if somebody could share experience I would very much appreciate it.

Thanks a lot and keep on rockin'[/quote]