Phonoland

MRA5-L6 amp recapped. Redhead with 0.7 mil needle producing very good volume and treble, but bass not as strong as expected. Bass increases when moved from #1 to #4 but increased bass level does not seem significant. Removing AVC increases bass a bit. Original speakers are in good condition. Speakers are wired in phase. Bass choke seems to be good. Caps installed followed values in Seeburg schematic "issue 3".
Suggestions for what should be checked to determine cause of low bass appreciated.

Hi Steve,
You could have other amp problems, or, if using the "new" needles, I have been told that they are VERY sensitive to needle pressure adjustments--Check your pressure, and counter balance spring adjustments. Ron Rich

Steve, you may have a bad output transformer.

That assumes that the rebuild was done properly and you have decent output from the styli. The presence of some gain increase when the 6Sk7 is removed would indicate adequate level.

An output tran with shorted turns will have poor inductance with poor bass and slightly lower output level.

Rob-NYC

Ron: Spring tension/needle pressure, and tone arm counterbalance appear ok. Cartridge/needle which performs well in my Seeburg C is being used to test the R.
Rob: how can output transformer be tested? I know resistance test is not reliable.

Hi Steve,
As far as I am concerned, the ONLY way to check the needle pressure is with a gauge. Once correctly set, it never needs to be done again--unless "someone" decides to mess with it---. Ron Rich

[quote="Steve_B"
Rob: how can output transformer be tested? I know resistance test is not reliable.[/quote]


Steve, if you don't have a scope and signal generator, the only remaining way to test is with a substitute tran.

If you have a audio frequency signal generator and an analog (needle type) meter with a db scale, you can get a good idea of teh output stage's response by doing the following:

-Remove the RVC jumper plug.

-Connect the generator to (IIRC) pin 7 and shield to chassis ground. Turn the AVC switch fiull-on (pos 4).

-Connect an 8 ohm non-inductive resistor across the speaker terminals on the amp. Or, you can connect you db meter across the speaker voice coil connections. the latter is a little less accurate because speaker impedance varies with frequency...but it will get you an idea.

-Finally, adjust the output of the generator for some convenient reading on your meter's scale that is well below the distortion point of the amp. then sweep through the lower frequencies. The amp should stay within about 2 db from about 500Hz to around 60 HZ at least.

These aren't the greatest output transformers and were prone to opening up in a primary winding.

I inherited a box of about 11 Seeburg output trans from op's closing up on Tenth Ave. Even though I have the necessary equipment to perform a ringing test, I found it more revealing to patch in the transformers one at a time on an amp i was rebuilding. I'd connect an 8 ohm load and test for power bandwidth and overall response. Some of the trans did show significant DC resistance between one side of the center tapped primary as compared to the other, but it was the actual powered test that really revealed a faulty transformer. Out of the 11 only about 6 or 7 were 'good".

If you find that the tran is defective I suggest a non-original transformer be subbed such as Hammond 1645:

http://www.hammondmfg.com/1608.htm

if you don't need the 70v output, the 1650F is probably a bit cheaper.

Bear in mind that the connections an mounting are often a bit different, but the results are worth the trouble.

Rob

Rob,
Would a low voltage into the output transformer from the 6L6's cause low bass, low volume, or both? I haven't measured the voltage yet but was wondering about the effect of low voltage.

Also, I did measure the resistance of the transformer even though I realize it provides limited information. The values from the center tap (red) to the other inputs were as follows:
Red/blue 225 ohms
Red/brown 201
Red/blue-red 49
Red/brown-red 56
Anything revealing about these findings?
Thanks

Steve, low voltage will indeed affect gain, output and bass. the input to the first filter should be around 400VDc.

The resistance number don't appear to be out of range (from memory), but they don't tell the story here.

Hope it is just voltage,

Rob