SCA35 "Tape-Head" makes HELLACIOUS noise

[Hi-Fi Low-Life]

I’ve just refurbished a Dynaco SCA35 and it sounds and looks great! It blows the doors off my T-Amp & Audio Analogue solid state amp. Yellow_Light_Colorz_PDT_06 Even-so, I’ll probably sell it on Ebay to pay for some other amp projects I’ve got in the pipe-line.

I want this thing working perfectly for its future owner, so before I sell it, I’ve got to figure out why, when the Tape-Head input is selected, a most gawd-awful LOUD noise is created in both channels. It’s far worse than just hum, I’d say the noise is around 100 hz and sounds totally over driven (and it’s no wonder, the pre-amp board delivers 10 volts AC to the volume control when Tape-Head is selected!!!) Shorting the Tape-Head RCA input jacks gets rid of this noise completely. BTW, it’s not the 12AX7s as I swapped in different tubes and got the same noise. Also, the phono pre-amp board is stock and its resistors measure fine. I don’t know how to measure the caps, but their impedances measured very high. This has always been an issue with this amp, so it’s not related to the refurbish work I did on the power amp boards. Finally, the Phono input works fine.

Is something wrong with the phono pre-amp board or was the Tape-Head input supposed to have shorting RCA plugs installed when not hooked up to an actual tape-head?

Thanks for any info any of you guys can provide!

[erichayes]

Hi All,

Most playback heads have pretty low DC resistance, HFLL, around 250Ω or so. This was enough to load down the grid circuit of the first preamp stage so it wouldn't hiss. Most preamps also came with shorting plugs in jacks that were liable to cause hissing problems if left open. The SCA35 would probably just hiss like hell if it weren't for those lovely little couplates they have on the low level inputs for EQ. The physical proximity of the resistors and caps on those things, along with the fact that they're all on the same substrate--ceramic!-- makes them oscillators waiting to happen.

Save yourself a lot of reengineering grief and use shorting plugs. If you're adventerous, you might try hanging a lowish value (10~20K) from the jack hots to ground and see what happens. This will affect the frequency response of the head preamp--if there's anyone out there besides me who actually uses head inputs anymore.

[Hi-Fi Low-Life]

Thanks for the info Eric. I assume the Tape Head input was meant for some sort of reel to reel format before reel to reel decks had standard line level pre-amps. Correct?

[sorenj07]

I think cassette tapes are also an option. "Tape head" is probably the wires straight off the little magnet, so you can think of it in the same vein as turntable cartridges - you probably need a fair bit of gain and possibly some equalization as well.

[TomMcNally]

Back in the late 50's and early 60's, all of the electronics catalogs
offered do-it-yourself tape decks that were basically just a transport
with a playback head. You could build or buy the electronics from
someone else. When I was a kid, my dad brought a reel to reel
tape recorder home for me that was mono, but with a stereo head,
and the other channel came out from a preamp stage to an RCA
jack. I modified the heck out of it, and had toggle switches that
I could listen or record on either track, or sum to mono.

Lotsa fun for a 12 year old.

[DuncanM]

As I recall, the "phono," "microphone," and "tape head" inputs all went through the phono preamplifier stage with different types of EQ applied (RIAA for phono, flat for microphone, and NAB for tape head).

The EQ switching on the first stage was not always flawless and one popular modification was to hard wire the EQ for the phono curve.

If you want to keep it stock, I agree that shorting plugs seem the best answer.

[EWBrown]

I'd SWAG, check that rotary switch and "suspect" the PECs. (Look for "Those Pesky PECs" in an earlier topic that I posted a couple years ago).

My factory-built SCA35 also screams, howls and roars with the TH and the phono inputs, since I don't use them, I haven't tried to correct it. Try the shorting plugs (a cheap RCA plug, wire the center and shell together, or wire in a 10K 1/4 watt resistor) or a just cut off, shorted cable will work.


That preamp board doesn't have the neatest and cleanest layout, and all those unshielded wires carrying audio signals, and those behind the front panel for teh tone controls, are just little antennas waiting to pick up any RF or EM interference that it can.

/ed B in NH

[kurtbauer]

Hi there,

I'm new to this site and am very interested in your SCA-35 progress. I have one too and have been considering making some more radical changes in the back. It may be too drastic for your needs since you may be moving on but here goes...

I was thinking of continuting to use the phono preamp but only for one input (an MM input which I guess would be the phono hi) and scrapping the rest of the LL inputs and the associated pesky PEC. That and changing the RCAs in the rear a la Van Alstine (and others including Curcio probably do).

Again this may be too drastic for your purposes. It may be too much for me too in that I'll have to REALLY understand what's going on in there including what's up with the wipers.

It's a thought anyway.

This is only one in a long list of things I'm considering for this unit -- it's not been well taken care of but the price was right! To that end is there a summary of possibilities for mods/rebuilds for the SCA-35? I've seen Roger W. Stevens site which is interesting. Other than that, I've been searching around and have seen dribs and drabs. I don't want to hijack this post so if there's nothing already done, I may try my luck at starting another thread.

Thanks for getting this far.

Best wishes.

[Hi-Fi Low-Life]

Hi Kurt,

You might consider this mod:

" if the original Dynaco 7199 driver is worn out and you replace it with a Sovtek 7199, the amp will oscillate at 500 Hz. This is because the feedback set-up in the SCA-35 is a bit quirky and only just stable with the original 7199, but not with the Sovtek which provides more gain. Detach the 1 uF C18 from pin 6, the pentode section's cathode, and connect it to ground instead. This will vastly lower the loop-gain and stabilize the amp once and for all. Plus, it will sound a lot better because of the much lower feedback."

As suggested, my SCA-35 did oscillate with the Sovtek 7199s, but this mod fixed that. I know a lot of people say uncomplimentary things about the Sovtek 7199, but the two I bought sound every bit as good as the RCA (Dynaco branded) 7199s that I have...

[kurtbauer]

Thanks, HFLL.

Yes, I had seen your reference to the Sovtek mod and actually found it on Audio-Note. I will definitely keep that in mind.

Also plan to study up on potentially relevant ST-35 mods. I may end up just upgrading connections, components, and such.

Already ordered some basic stuff.

Be sure to give us a heads up when you put yours up for sale. I'd like to recommend it to some friends.

Kurt