Archive for tube

Gates Sta-Level

Posted in Repaired, Restored with tags , , , , , , , , , on January 22, 2014 by AnalogAudioRepair

A client gave me a beautiful original Gates Sta-Level to bring back to life.  It had all original components but had not been turned on in over ten years.  After testing all the tubes, I brought it up on my Variac with just a solid state rectifier installed.  The power supply filter caps were good, but not great so they got replaced first.  With the new PSU caps and the tubes installed the unit was working ok, but had a very strange frequency response.  Above 1kHz it was fairly even, though not quite up to spec.  Below 1kHz it increased steadily to about +12dBm at 30Hz!  It turned out the feedback caps from the 6V6 output stage to the 12AU7 were leaky.  I replaced those and the coupling caps and things are now back to factory spec.

Vintage Gates Sta-Level

Original Gates Sta-Level

Gates Sta-Level original guts

The guts – dead stock!

Gates Sta-Level power supply re-cap

New Power Supply Caps

Gates Sta-Level capacitors

Replaced Film Caps

 

Neumann CMV-563 Recap

Posted in Repaired with tags , , , , , , , , , , , on December 3, 2013 by AnalogAudioRepair

The Neumann CMV-563 microphone body can be used with different capsules including the famous M7 (cardioid) and M8 (figure eight).  This unit was getting noisy and swapping the tube did not entirely solve the problem. The two critical capacitors are a polystyrene cap which couples the capsule to the tube, and a film cap connecting the tube to the output transformer.  We decided to replace the third cap as well, although it was not original (green chicklet cap in the Before photos). The polystyrene cap requires extra care. A heat sink during soldering prevents damage to the component, then everything in this high impedance area must be carefully cleaned.  Any flux or oils from fingers can cause noise.  The other two caps were replaced with Wima film caps, increasing the size of the output cap to 1mfd. This was a really tight fit, but worth it. In the end, with a fresh NOS tube (not shown) the mic sounds great and the noise floor is much lower.

Before - cheap replacement cap

Before – cheap replacement cap

original Neumann CMV 563 polystyrene capsule coupling capacitor

Original polystyrene cap

 

Heatsink for Neumann CMV 563 tube microphone capacitor replacement

Heatsink on the new polystyrene capacitor

Neumann CMV 563 vintage tube condenser microphone output coupling capacitor

Original output coupling capacitor

Neumann CMV 563 vintage tube condenser microphone Wima output coupling capacitor

Wima 1.0uf output coupling capacitor

Vintage Tube Mixer Direct Ouputs (RCA, Ampex, Altec, Etc.)

Posted in Modded with tags , , , , , , , on July 30, 2013 by AnalogAudioRepair

Not a lot of great pictures this time, but something I do get requests for.  There are quite a few nice tube mixers around that sum two or more inputs into a single output.  Often folks would like to use these as individual mic preamps with their own outputs.  Your only choice is to use them as a single channel pre, or add some sort of direct out.

The issue is that these preamps use a single tube (or half of a dual triode) to amplify each input, then one or more tubes for EQ and output stages.  Of course it would be costly to add identical output stages to each channel, even if the power supply could handle it.  So the quick and dirty method is to take an unbalanced signal right off the coupling capacitor from the first stage.  The downside is that these outputs can’t drive much of a load, and have a reduced level compared to the main outs.  The upside is that they generally work well into the high impedance inputs of most converters and other modern gear.  Also, by using a switching jack, the signal can be connected to/disconnected from the mix buss automatically.

I have done this recently with a vintage RCA mixer (MI-C38690) and an Ampex MX-35 and both clients were quite pleased with the results.  The RCA also got input pads and a nice panel for all the connectors as well as some power supply cleanup.  The Ampex was additionally modded with line inputs on the first two channels.

RCA tube mixer with input pads and direct outs

RCA tube mixer with input pads and direct outs

RCA Tube Mixer Wiring

RCA Tube Mixer Wiring

RCA Power Supply Mess - Before Servicing

RCA Power Supply Mess – Before Servicing

Ampex MX-35 Line Inputs

Ampex MX-35 Line Inputs

Ampex MX-35 Direct Outs

Ampex MX-35 Direct Outs

Ampex 601 Tube Preamp & DI Mod

Posted in Modded, Restored with tags , , , , , , on March 22, 2013 by AnalogAudioRepair

This was a fun project that involved removing some tape deck electronics and modding the remaining circuit into a tube mic preamp with DI.  As you can see, several of the tubes are not needed when using this as a preamp.  A whole bunch of other parts came out too which cleaned up the crowded underside of the chassis a good bit.  The headphone jack is now the DI input and the Input/Tape switch selects either the mic or DI.  The Line Rec Level pot is now the DI level control and an XLR output jack has been added on the side.

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Telefunken V72S Transformer Replacement

Posted in Repaired with tags , , , , , on February 26, 2013 by AnalogAudioRepair

This is a rare Telefunken V72S – the “S” is a higher gain version.  Unfortunately this one had a bad input transformer.  The frequency response of the transformer looked like this: /   A rebuilt original was too costly for my client, but Sowter offers a replacement that works just fine.  The hard part is removing the original and mounting the new one!

RCA BA-2C Tube Preamp

Posted in Restored with tags , , , , on February 26, 2013 by AnalogAudioRepair

This vintage RCA vacuum tube preamplifier was missing it’s power transformer.  Fortunately I had the schematics and a transformer that would fit the bill.  A couple power resistors were required to get the voltages spot on, but in the end it worked out well.  The unit got a full re-cap and a fresh set of New Old Stock vacuum tubes.  The new owner even received an original manual!

Fender Re-Issue Amp Repairs

Posted in Repaired with tags , , , , , , on December 29, 2012 by AnalogAudioRepair

I recently serviced a pair of Fender Re-Issue tube amps from the 1990’s.  One was a Deluxe Reverb with a shorted output tube, the other a Vibrolux with a blown cap (literally).  These ’90s re-issues are much like their predecessors, but with printed circuit boards which make it a bit more of a pain to work on them.  At least the Deluxe has connectors for the front panel controls rather than soldered wires like the Vibroverb!  It had some 2 watt resistors that were so charred they almost burned through the PCB.  I had to scrape away as much carbon as possible and seal the board with epoxy before replacing the resistors and power tube.  The Vibrolux had a capacitor which actually burst and both amps had very weak Chinese made 12AT7s.  While I was in the Deluxe I added the de-clicking cap to the vibrato circuit.