Silvertone 3902 P/P 6BQ5 Console amp

[Pio1980]

Over-all at this point I'm quite impressed! I don't recall my reworked Dyna Stereo 70 sounding this transparent and "honest" much less the Scott 299C, both long gone.

 

[Horn Head]

Very much enjoyed the listening sessions at Hornheads place, haven't done audiophile GTG's since the mid-'80s and miss that. The cornerhorns have magnificent bold large presentation that serve the Vangelis large scale works well but are not the last word in neutrality or clarity, his secondary system MTMs are very precise and neutral and would be worth moving into the tube set-up on stands for a time IMO for a comparison.

Those homebrew MTMs?? You know I have them hooked to the SX-1080 to provide sufficient power....Don't hink there's anyway these little PP amps could move those little cones suffuciently; bet they're only like 85dB efficient. Heck, my JBL 4312as didn't fare well with the PP maggie at anything over conversation level sound.

Bigger horns...better room necessary...

Pio Sorry I missed you call....was knee deep in a unplanned chemical stripping project when you called and was barking out orders to the whole family to assist or go away until I could complete the project....before losing a fine piece of furniture. Will try to call tomorrow. I'm sure you have some goo updates to discuss. (Wasn't trying to be rude....was just an intense time when you called. I think my boy handled it well though.)

 

[Pio1980]

It's OK, no prob, I know your situation. He handled it fine. Was going to see about setting up another GTG.

I didn't expect the Advent/2 Min-E-Waves to do anything on the little Silvertone (Advents notoriously prefer a current source S/S amp) but it really makes them come to life. I didn't honestly know how good they were until this amp happened and I wasn't running them off garbage. Clarity and great dynamic scaling apparently makes up for the relative low power.

 

[Pio1980]

A little bit more tidy-up and a couple noise snubbers.
Another session at Hornhead's under the belt and the cornerhorns were sounding much better to me. The revised amp was controlling the bottom end noticably better along with good mids and highs in proper proportion. All in all a delightful final afternoon's shared auditioning.

 

[Horn Head]

Yep, second that. Immediately upon firing it up, noticed what seemed to be a more controlled low end and better balance with the mids/highs. Sounded spot on and no criticism whatsoever (other than it's not mine anymore!)

Before all PIO's mods, especially those to the PS, bass seemed a bit heavy compared to to my maggie 8802 and other amps. Today proved a noticeably different sound from its stock form.

Glad that PIO explained those PS mods to me so can duplicate similar ones on the other amps sitting around. All this electronics stuff may be starting to sink-in a bit. And a shout out to Battradio who I believe is feeding this project some good info.

Edit: And interestingly, Power Transformer heat was reduced from 133 F at last session to 127 F this session (due to the power supply mods?)

Nice to see PIO warming up to the cornerhorns...

Now just need to build an extension off your house so you can get a pair for yourself....or maybe some Altec A-5s!

 

[battradio]

Over-all at this point I'm quite impressed! I don't recall my reworked Dyna Stereo 70 sounding this transparent and "honest" much less the Scott 299C, both long gone.

Hi Steve ,

Most ST 70's never have enough power supply stiffing and yours still had the 7199 driver in it . I have two st 70's to modify and want to do one with fixed bias and the other with cathode bias and belive the one with cathode bias will sound more like your amp .

 

[kb8dns]

I have a similar amp inherited from my dad, it is mono and has 6V6s but the sound is, like you said, "vivid" ...... :thmbsp:

I have been looking for a second but nowhere to be found....

Rod

I'm apathetic, but I don't care

 

[Pio1980]

My ST 70 was also running EL34s instead of 6CA7s. Using those with a front end sim to the Silvertone or Fisher 30A along with the PS reservoir mod could make it something special.

 

[Horn Head]

Hey PIO,

Got the soldering iron out last night and went straight for the PS on the Maggie 8802 per the little drawing from Saturday.

Didn't install that first cap (the one not on the schem), but raised values for the others; 1st 40 to 130uf, 2d 40 to 90uf,and 3d 20 to 47 uf. Also swapped the 10k R3 from 10k to 22k. Had to add several caps to the monster Atoms in there already - space was getting tight. It's not pretty but it's functional. (Will clean-up later.)

Also made those standard mods for coupling caps from .0047 & .047 to .0095 (.01 effectively) and .1 uf respectively.

Didn't touch C2D cap at 30 to raise to 470uf yet.

But....this 8802 started performing with the bottom end authority your Silvertone presents. Listened about 5 hours last night and about 4 more tonight and I think it's settling down. (And mind you, I was a hard skeptic on the whole cap break-in thing).

Went through the test CD and it's sounding very good. Got audible sound out of the 31.5 test tone! Then nothing but flubber below that.

Thanks for the tips. I think we have improvement on another amp....and some lessons learned on this end from your excellent explanations about stiffening the PS.

So excited I just might order some of those Edcor Xfrmrs Battradio recommends for these. Bottom end is getting tighter and the soundfield is closing-in on those wide cornerhorns.

Thanks for the inspiration!

Hope the Silvertone is continuing to please....

 

[Pio1980]

Things should get interesting when you get the plate reservoir mod in. Do also consider the power supply upgrade for your amp head, leave that amp's cathode resistor un-bypassed (no cap).

Listened to some vinyl thru the Silvertone tonite via the tape out sockets (to bypass the pre's line section) from a Sansui C 2102 pre's phono section thru the passive control. I'm planning a tube RIAA phono pre for this set-up soon. I do want to firm up the amp's upgrades with some follow-up later, it's pretty much 'there' now otherwise.

 

[gogofast]

hello guys,

can you possibly list all the things you've done in that silvertone amp? do you just disconnect all those ac out wires to tuner/pre turntable? how did you work the audio-in? and power switch? which color wires? i have the same amp sitting in the garage for a few years....it'd be nice to make it sing. thanks!

 

[yungin's-dad]

Dudes, Reading these threads is like watching that bar scene in the Starwars movie where all types of languages were being spoken.....and you're the guy who doesn't understand any of them. Hmmmm, maybe someday with enough reading and coaching.

v/r, HH

I haven't finished reading this thread yet, but you seemed to meet minds with me about here. My version is more like watching Charlie Brown when his teacher is speaking!

Pio, I can how ever understand Un Assing the Area of Operation though!!

Great thread! And I'm sure even better listening for you HH.

I will continue on now.

Ron

 

[yungin's-dad]

A little bit more tidy-up and a couple noise snubbers.
Another session at Hornhead's under the belt and the cornerhorns were sounding much better to me. The revised amp was controlling the bottom end noticably better along with good mids and highs in proper proportion. All in all a delightful final afternoon's shared auditioning.

Wow Pio, I have to say, that's some great work there!!! That thing is fortified like a brick **** house! I have been thinking about picking up a little Magnavox or something like that. Now you have me REALLY wanting one.

Thank you guys for the great entertainment!! :thmbsp: Not so much for the desire I now have to buy another amp. :gigglemad

Ron

 

[yungin's-dad]

Those homebrew MTMs?? You know I have them hooked to the SX-1080 to provide sufficient power....Don't hink there's anyway these little PP amps could move those little cones suffuciently; bet they're only like 85dB efficient.

HH, can you tell me about your home brew MTM's? I have a pair of them that were built off of the John Krutke's (Zaph Audio) design. Vifa XG18 mid woof's and Seas 27 TDFC tweet's. They are actually very efficient, and are now my speaker's of choice in my tube rig. IMHO, they knock the crap out of my Forte II's, in fact they are the best sounding speakers I've ever owned solid state or otherwise bar none. You really should take Pio's advice and give them a shot.

Just one of my many observations in this thread.

Ron

 

[Horn Head]

HH, can you tell me about your home brew MTM's? I have a pair of them that were built off of the John Krutke's (Zaph Audio) design. Vifa XG18 mid woof's and Seas 27 TDFC tweet's. They are actually very efficient, and are now my speaker's of choice in my tube rig. IMHO, they knock the crap out of my Forte II's, in fact they are the best sounding speakers I've ever owned solid state or otherwise bar none. You really should take Pio's advice and give them a shot.

Just one of my many observations in this thread.

Ron

Ron, The MTMs were a complete accident. Threw some inexpensive drivers together with some 'gut' feeling box design and a single cap x-over....and turned out OK.

See this thread http://www.audiokarma.org/forums/showthread.php?t=305947

They're not efficient, so not advantageous for low powered tubes. But on the SX-1080 - very nice with a sub.

And now that you mention it, I haven't actually played them off a tube amp. May do so just out of curiosity. Will fire them up on the Magnavox 8802 hopefully this week.

Finally, yes, PIO did a fantastic job on the Silvertone. The power supply mods were the most significant to my ears....a must do for anything similar.

v/r, HH

 

[Horn Head]

hello guys,

can you possibly list all the things you've done in that silvertone amp? do you just disconnect all those ac out wires to tuner/pre turntable? how did you work the audio-in? and power switch? which color wires? i have the same amp sitting in the garage for a few years....it'd be nice to make it sing. thanks!

GOGO, PIO will have to answer that one as my memory has diminished. I do remember getting many sparks as I tried to jump the correct leads from the original molex connecter...actually sizzled one of the big resistors at the power input while trying to get it right. Luckily didn't fry anything important. Hopefully he can tell you which colored wires are connected in which way....

The speaker and input connections were pretty simple though. Just routed the common (ground) and L&R inputs to some RCA jacks mounted on the rear of the chassis.

Same with the speaker terminals. Make sure the common is grounded to the chassis as well as the negative speaker terminal.

 

[gogofast]

GOGO, PIO will have to answer that one as my memory has diminished. I do remember getting many sparks as I tried to jump the correct leads from the original molex connecter...actually sizzled one of the big resistors at the power input while trying to get it right. Luckily didn't fry anything important. Hopefully he can tell you which colored wires are connected in which way....

The speaker and input connections were pretty simple though. Just routed the common (ground) and L&R inputs to some RCA jacks mounted on the rear of the chassis.

Same with the speaker terminals. Make sure the common is grounded to the chassis as well as the negative speaker terminal.

thank you for your reply! i'll go ahead and work on those first.:thmbsp:

 

[Pio1980]

The upgrades I did were mainly capacitor replacements for the ceramic disc caps and the aforementioned power supply revision. I have no service info or circuit diagrams for this amp.
Good soldering skills and some understanding of the nature and appreciation of the danger of working on/with higher voltage electronic apparatus is essential to do this. Many with care and little experience have done it with help form these forums.

Refer to underside chassis photos earlier in the thread;
There are six .02 uF disc capacitors, the two at the input points are discarded and bypassed with a direct connection from the respective 12AX7s to the added RCA input sockets and the four others swapped for 0.1uF/ 400V metallized polypropylene, tho' polyester or other type plastic film caps are OK. The two 150pF/mmF caps and 2 47pF/mmF are replaced with 400V plastic film caps of the same values.
The two small ca 22uF/25V electrolytic caps under the chassis are replaced with 470uF/25V (pulse power supply rated preferred) electrolytic caps, mind the polarity! Bypassing these with a ca 0.47uF/60-100V film caps is a good option but not absolutely necessary. The 4700 ohm 2 watt resistor on the tall silver can electrolytic cap should be replaced with a 10k /2 watt resistor. With this one exception all resistors should measure within 10 percent of the indicated values. The four 100k resistors, the two 2700ohm cathode resistors, and the two 270k plate resistors associated with the two 12AX7 input tubes are most critical to proper operation along with having good 12AX7s and must measure within 10 percent, the others not so much. The electrolytic caps not known to be good should be replaced, there is a procedure for checking them to be described on request. The main power supply mod consists of adding a 100 ohm 5 watt resistor between the 1000ohm square-bar resistor and the capacitor lug it was formerly connected and moving the 2 red wires from the lug to the point between the two resistors. A plate reservoir capacitor of 220-470uF/400V is added to that point with the negative end of that cap to chassis ground return. at a convenient point. An additional cap of ca 100uF/400 volt is added to the other end of the 1000 ohm square-bar resistor with it's negative to a chassis ground. A 0.1uF/400V plastic film cap is paralleled with the 100uF capacitor at a convenient tie point and another is added to the other end of the replaced 10K 2 watt power resistor at a convenient point with the other end to chassis ground. I cut off all the plastic molex wires except the white ground wire and the black and green cloth power wires at the chassis terminals and the black and green cloth braid wires at the molex plug. The green cloth wires from the power transformer are carefully insulated from each other and the chassis before dressing out of the way, they are unused. The black cloth wires and black plastic power wire are used for the switch and fuse & power cord. There are several versions of this amp with different umbilical/molex wiring, regardless do not cut off cloth wires at the chassis/transformer.
Noise snubber capacitors across the power switch and the power input are a good idea, these are specially rated versions. I used a couple older style large ceramic disc capacitors visible in the revision pic.
More later as needed/requested.

 

[gogofast]

thank you pio!
i'm gonna have to print your post and study it.

 

[gogofast]

found a schematic and parts list - scanned it. here you go. maybe it'll be helpful for your further mods.