Please end the Audio Capacitor debate - Page 1
Forum section: electronics / repair

author: retrotubeguy
author_rank: Contributor
timestamp: September 10, 2015, 08:03:39 am
content: Can some on here help settle what i am sure is an ongoing debate. These old caps were apparently Polyester, Polyethylene Terephthalate (PET). Can they be replaced with modern polyester film or metalized film. There is some debate about metalized film but aren't they all metalized in some way. The caps I need to replace regularly are directly in the audio path. I have had no problems with polyester caps in this application but i am sure there is a better cap for the job. possibly a more transparent type, Orange drops are the obvious choice but they come in limited values and they are sometimes to large to fit in the units I repair. Don’t want to alter the tone of the units too much or at all if possible. I assume values are more important in this application. Some of these caps are on the anode B+ rail of the preamp tubes so they are around 270 to 38vdc. the AC amplified audio signal is also on this same rail.Just hoping someone can settle the debate so i can move on.Thanks

author: Karel
author_rank: Super Contributor
timestamp: September 10, 2015, 08:36:46 am
content: They will work fine. However, for audio and lf-filters, the best are polypropylene capacitors.For example this one:http://www.newark.com/wima/fkp2d011001d00hssd/film-capacitor-1000pf-100v-radial/dp/46T6646

author: BradC
author_rank: Super Contributor
timestamp: September 10, 2015, 08:38:48 am
content: To be honest given the biasing on the input amplifiers, the "transparency" of any of the capacitors in the circuit is irrelevant as the circuit leaves any pretense of linearity or accuracy at the input jacks. Grid leak biased circuits do sound beautiful though and the asymmetrical impedance that presents to a guitar pickup is one of those things of beauty nobody seems to ever have got right in silicon.I'd go with whatever you can find that meets the voltage ratings of the various sections. Any plastic in the signal path is going to sound better than a ceramic, but arguing over which sounds best in a magnetic echo is kinda like tube rolling. The best sound will disappear behind a mask of non-linear magnetic noise.Out of interest is this a tape or a drum? It kinda looks a bit like a Binson, but there were a gazillion clones.

author: retrotubeguy
author_rank: Contributor
timestamp: September 10, 2015, 11:14:20 am
content: Thanks guys, That's kind of what i was thinking all along. Any cap will cause some sought of distortion, I just did not want to stray to far away from the original distortion characteristics. FYI This is a Vox Long Tom CO2 Deluxe MKII. It is a tape loop. I removed some details from the schematic as i did not have permission to share it. Title block removed and switch mode labels removed record erase bias coil details removed. Schematic courtesy of Charlie hall. If some one genuinely needs the full schematic I can email it to them.

author: BradC
author_rank: Super Contributor
timestamp: September 10, 2015, 11:22:30 am
content: Charlie is a bit of a guru. I love his EFTP patches on my Quadraverb.I did wonder when I saw it had 6 replay heads. Quite a unique echo on that one.My old man has the last Echomatic IV, so an "old" valve based tape echo but made about 3 years ago. Lovely sound, but high maintenance.

author: retrotubeguy
author_rank: Contributor
timestamp: September 10, 2015, 11:43:53 am
content: Yes Its great, this one belonged to my dad. He took it to a TV repair guy in the early 70's who replaced the 5k-5w power resistor in the rec erase bias circuit with a 1k-5W, He also decided that a 6EM5 was better than an EL 84 and that a 6BU8 would be better than a EZ 80 rectifier. whiles he was at it he decided that Dick Denny did not know what he was doing and replaced all of the preamp tubes with a 12Au6 instead of a 12 AU7; hey its only one digit. He replaced all 12 AX7 with 12AW6, because 4 electrodes is better than 3. It has been a labour of love getting this back up and running. Just waiting on new tubes and some electrolytics and hopefully its done. I will post a video if i get it running. Hoping the oscillator coil and erase head survived that resistor mod.

author: retrotubeguy
author_rank: Contributor
timestamp: September 10, 2015, 11:57:25 am
content: And yes i realize that if a pentode was put in place of the rectifier no DC voltage would be present to do any circuit damage to the erase/rec bias section. But did he power it up with a good rectifier and then change it to a pentode. we will never know. So its all a bit up in the air. An know I am not going to pull the coil out or try to isolate it and measure it. It is surrounded by silvered mica caps not cheap & the coil is pure unobtanium. Trust me I have been looking for 20 years. Still looking. I can get something similar but not the same. if I am forced to i will wind my own.

author: TimFox
author_rank: Super Contributor
timestamp: September 11, 2015, 07:18:58 pm
content: How do you replace a 12AU7 (9-pin base) with a 12AU6 (7-pin base)?

author: retrotubeguy
author_rank: Contributor
timestamp: March 08, 2016, 08:06:11 am
content: In replay to an older post by Tim Fox, I just saw this.Tim asked; How do you replace a 12AU7 (9-pin base) with a 12AU6 (7-pin base)?My rather late reply, Sorry I forgot to mention that the repair guy changed the sockets. what a mess.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 08, 2016, 10:10:51 am
content: Quote from: retrotubeguy on March 08, 2016, 08:06:11 amWhat the hell was he smoking?

author: Brumby
author_rank: Unknown
timestamp: March 08, 2016, 11:40:17 am
content: I'm guessing tag strips - and possibly an output transformer....

author: vk6zgo
author_rank: Super Contributor
timestamp: March 09, 2016, 01:13:00 am
content: Quote from: Brumby on March 08, 2016, 11:40:17 amTha's real good shit,man!!!

author: jh15
author_rank: Frequent Contributor
timestamp: March 09, 2016, 04:52:45 am
content: Make sure you burn in the tag strips.