Saturday, December 29, 2012

Marvel Amp Update

I dropped by Bruce Collin’s Mission Amps shop yesterday for a little work on the newly acquired Marvel vintage tube amp. This was the first time I’d had the chassis all the way out of the amp. The circuit is sparse and primitive, reminding me of a 1947 Gibson amp I once owned. It has a half wave rectifier, just a tiny diode. Weird.

Mostly I wanted Bruce to install a grounded power cord and test the caps, because I really like the tone of the ratty little amp as it is. Here is what we ended up doing:

-Install grounded power cord
-Remove the “Death Cap”
-Install cathode bypass cap on the power tube. This gave the amp a bit more punch.
-Test all tone caps for leakage. 
-Test all circuit voltages.
-De-oxidize the tube sockets
-Install speaker jack

We then tried five different preamp tubes: 

-The vintage RCA gray plate 12AX7 I had in the amp
-a 5751 from JJ Tesla
-a 12AY7 from EH
-an NOS 12AZ7
-an NOS RCA 5965

Of those tubes I liked the 12AZ7 best, so I have it in the amp for now. We did not change the power tube. The original 7591 tube is a gem; a Sylvania rebranded as Multivox.

We found some date codes on the chassis and filter cap. It looks like it was made late in 1963.

The amp tested at 7 watts just as it began to clip. It still has the same old-school raspy sound but slightly browner. Sounds good! It’s a keeper.

Friday, December 28, 2012

Vintage Kay Amp

My friend Timothy Patrick O'Reilly from the DC area sent photos of his cool vintage Kay amp.

All the tubes are original and Tim says it sounds great.  That is a 6L6 power tube on the left and a tube rectifier on the right.  A 12AX7 preamp tube is hidden in the far left.  With that 6L6 I bet this little amp kicks butt.  I'd like to hear it next to my little Marvel amp.

Tuesday, December 4, 2012

Test - Amp volume vs Mic volume

This video compares two scenarios:  In the first segment the volume on the amp is all the way up and the volume on the harp microphone is barely cracked.  In the second segment the amp volume is on 4 and the microphone volume is all the way up.

My phone is in the foreground running the RTA app to show the relative loudness. The amp is a Mission Chicago 32-20 and the mic is a 1959 Shure 440SL with 99B86 CM element.

The two segments sound similar.  To my ear the segment with the amp all the way up was less lively and not as full sounding.  The amp tipped into screaming feedback if I turned the mic up the slightest bit, so this is about as loud as I could get in this scenario.  That was my benchmark volume for the comparison.

In the second scenario the amp is on 4 with lots of room for more volume, and a greater range of tonal inflections.

I'd heard about the "amp all the way up" thing over the years and decided to test it after hearing a jammer do it at Ziggies the other night.  I think there is a good reason it is not very common.  The traditional way sounds better.

Please pardon my noodly playing....