Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

DIY Tube Preamp causing Power Amp Protect Mode
DIY Tube Preamp causing Power Amp Protect Mode
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 20th August 2019, 06:36 PM   #1
joshvito is online now joshvito  United States
diyAudio Member
 
Join Date: Jun 2019
Exclamation DIY Tube Preamp causing Power Amp Protect Mode

I built a single ended tube preamp (the circuit is here Sofrito Preamplifier – wauwatosa tube factory). The preamp seems to be working great. By great, I mean that the sound it produces is clean and clear with a bit of :tube:.

Circuit Schematic:
Click the image to open in full size.

My build has 2 minor differences:
1. I am using 4.7uF output caps
2. I added a DPDT switch in between the rotary selector switch and volume pot. The inputs ground run to the volume pot and then to the star ground at the output tube (6V6).

However, my amplifier kicked into protection mode the other day about 30 mins into a listening session.

What could cause my amp (Marantz PM8005, input sensitivity is 1.6v 15kohm, running in Power Amp Mode) to go into protection?

Is the addition of my newly built preamp just coincidental? The system was working with out event for ~7 two hour listening sessions. The last session, I got about 30 minutes through a record before the amp went into protection mode.

I removed the DIY preamp from the system, and using the built integrated preamp of the Marantz, and the second half of the record played okay.

What have I tried:
1. I tested the DC voltages from the power supply (after shunt): 330 vDC
Voltage at the 6v6 is 220 v DC as I expect from the load lines in the linked blog post above.

2. While powered on, with no input signal and no output connected, I measured the output RCA ground with multimeter attached to output ground and input ground. DC and AC voltages were 0.
I also measured output RCA ground with meter attached to output ground and chassis earth. This was also 0 (actually, meter read 0.1mV).

When I leave the multimeter attached to output RCA ground and output signal, with no input signal, DC voltages jump all over on my auto ranging MM. Range about is +- 200mV DC

Here is a video of what I am talking about.
YouTube


Other information:
I have a lot of build pics at DIY Stereo Pre-Amplifier Build (Sofrito clone) - Album on Imgur

Here are a couple images so you don't have to scroll through all the build pics:

Power supply section, rectifier tube on top, shunt on bottom
Click the image to open in full size.

Preamp section:
Click the image to open in full size.
Click the image to open in full size.

Thanks for any help, as I am new to the DIY audio space.

I am reluctant to put the unit back into my main system until I think it has been tested a bit more.

PS. Sorry if you saw a similar post on AK, as I asked for help over there too.
  Reply With Quote
Old 21st August 2019, 04:14 AM   #2
mashaffer is offline mashaffer  United States
diyAudio Member
 
Join Date: Jun 2004
Location: Indiana
Was thinking the tubes/valves subforum would give you more exposure.
__________________
If you are not a part of the Solution... you are a part of the Precipitate
  Reply With Quote
Old 21st August 2019, 04:51 AM   #3
leadbelly is offline leadbelly  Canada
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
It would help a great deal if you could mark up the schematic to match what you built. I think I see 2 6V6 and 2 6L6, is that correct? How are the 6V6s triode strapped? No 5U4? SS diodes? Is it dual mono in one chassis or does 1 supply split into 2 regulators at some point? How exactly do the rotary and DP switches wire up?
__________________
Wise men speak because they have something to say; Fools because they have to say something. Plato
  Reply With Quote
Old 21st August 2019, 09:46 AM   #4
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Could it be mains voltage variation? At first glance it appears that there is a band of frequencies in the subsonic region which will be passed by both the PSU and the line amp output, so any change in mains voltage in this bandwidth will be passed straight through to the power amp input.
  Reply With Quote
Old 21st August 2019, 10:04 AM   #5
mashaffer is offline mashaffer  United States
diyAudio Member
 
Join Date: Jun 2004
Location: Indiana
BTW, I don't think we ever confirmed how the 6V heaters are referenced to ground. Is there a grounded CT for them or did you make a false CT?

Somebody check me but that rectifier wiring doesn't look right to me.
__________________
If you are not a part of the Solution... you are a part of the Precipitate
  Reply With Quote
Old 21st August 2019, 03:06 PM   #6
jean-paul is offline jean-paul  Netherlands
diyAudio Moderator Emeritus
 
jean-paul's Avatar
 
Join Date: Sep 2002
Location: Germany
Please use a muting circuit with a relay at the output of that preamp to prevent blowing up speakers when the preamp is switched on or off when the power amplifier is still switched on. This counts especially with DC coupled amplifiers but it is simply good practice in any amplifier case. It should release outputs after 30 seconds at power up and it should short the outputs immediately at shutdown. Series resistors 47 Ohm from each of the 1 megaOhm resistors to the relay contacts can make you feel better.

It would also be nice to put simple RC filtering at the inputs to prevent HF/RF straying in. Tube amps are quite robust but solid state amps are a bit more sensitive (in general).

Regarding the issue: could it be that the 6L6 is oscillating?
__________________
It's only audio. Member of the non modular PCB design committee

Last edited by jean-paul; 21st August 2019 at 03:27 PM.
  Reply With Quote
Old 21st August 2019, 03:10 PM   #7
joshvito is online now joshvito  United States
diyAudio Member
 
Join Date: Jun 2019
Quote:
Originally Posted by leadbelly View Post
It would help a great deal if you could mark up the schematic to match what you built. I think I see 2 6V6 and 2 6L6, is that correct? How are the 6V6s triode strapped? No 5U4? SS diodes? Is it dual mono in one chassis or does 1 supply split into 2 regulators at some point? How exactly do the rotary and DP switches wire up?
There is one power supply that is feeding a stereo pair of 6V6 (as triode). To run the 6V6 as triode, I shorted screen and anode pins (pins 3 and 4). Signal enters the 6V6 on the grid pin (pin 5). B+ (220v DC) is fed onto pins 3/4.

I posted a schematic in the thread to illustrate the wiring of the inputs and switches. Sorry for my rudimentary drawing in advance.

There is one 5U4 rectifier, one 6L6 used as shunt, and 2 6V6 (one for each channel).

Quote:
Originally Posted by mashaffer View Post
BTW, I don't think we ever confirmed how the 6V heaters are referenced to ground. Is there a grounded CT for them or did you make a false CT?
The 6.3v heater and primary have center taps on the transformer. These are grounded in the power supply section at the negative of the first filter cap.

The 5v AC winding from the transformer feeds the 5U4 rectifier heater.

Quote:
Originally Posted by jean-paul View Post
Please use a muting circuit with a relay at the output of that preamp to prevent blowing up speakers when the preamp is switched on or off when the power amplifier is still switched on. This counts especially with DC coupled amplifiers but it is simply good practice in any amplifier case. It should release outputs after 30 seconds at power up and it should short the outputs immediately at shutdown. Series resistors 47 Ohm from each of the 1 megaOhm resistors to the relay contacts can make you feel better.

It would also be nice to put simple RC filtering at the inputs to prevent HF/RF straying in.
These sound like great upgrades to the circuit. I was handling the former by powering up the preamp first and powering down the preamp last.

Do you have any favorite documantation for the above circuit changes. I am new to this, and there is a wealth of information. SOmetimes it is challenging to understand who/what to trust from a google search.

If it helps, the transformer I am using is Hammond Manufacturing 6K7VG

Quote:
Originally Posted by jean-paul View Post
Regarding the issue: could it be that the 6L6 is oscillating?
How can I check that? DC voltage from the 6L6 is steady on my multi-meter (MM).

Would that measure as AC? How would I connect the leads of my MM to measure possible AC at the output of the 6L6?

Quote:
Originally Posted by mashaffer View Post
Was thinking the tubes/valves subforum would give you more exposure.
Is there an easy way to cross post this thread over there, I'd hate to duplicate the thread. Are there any forum precedent for this?

Sorry about posting this link a couple times, but I cannot edit my previous replys
Click the image to open in full size.
  Reply With Quote
Old 21st August 2019, 04:42 PM   #8
indianajo is offline indianajo  United States
diyAudio Member
 
indianajo's Avatar
 
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
You need to check for ultrasonic oscillation. I had 1 mhz @ 1vac in a mixer I sped up the op amps on, and it sounded okay, but the power amp hated it.

If you don't have a scope, an analog AC voltage multimeter 5000 ohms/volt with 2 vac & 20 vac scales will work. Block the negative probe with .047 600v to prevent the AC scale from reading on DC. If you have output with no sound, very suspicious. You can prove any AC voltage out is ultrasonic by changing the blocking cap from .047 uf to 390 pf. Audio won't pass through 390 pf, but ultrasonic oscillation will.

a 5 v protection diode across the output would prevent huge spikes from destroying your power amp. You also need 33 pf disk caps from center to ring on each input connector to prevent broadcast radio, police/fire/cb band, or cell phone RF from getting into your circuit. You do have a safety grounded metal box don't you? If not, start at the very beginning, a very good place to start. If you have exposed tubes outside the metal case, use ground clip tube sockets and put metal caps over them to prevent RF pickup. Hammond organ did on the H100. Not everybody had a radio transmitter (cell phone) in their pocket in 1930 but many did by 1965. alternately you can put a grounded mesh box over the top to let the hot air out and keep the RF out.
__________________
Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300

Last edited by indianajo; 21st August 2019 at 04:54 PM.
  Reply With Quote
Old 21st August 2019, 05:16 PM   #9
leadbelly is offline leadbelly  Canada
diyAudio Member
 
leadbelly's Avatar
 
Join Date: Dec 2002
Location: Calgary, Alberta
I would put grid and screen stoppers on the 6L6 and 6V6s. Make sure soldered right to socket.
__________________
Wise men speak because they have something to say; Fools because they have to say something. Plato
  Reply With Quote
Old 21st August 2019, 05:39 PM   #10
mashaffer is offline mashaffer  United States
diyAudio Member
 
Join Date: Jun 2004
Location: Indiana
Quote:
Originally Posted by joshvito View Post
Is there an easy way to cross post this thread over there, I'd hate to duplicate the thread. Are there any forum precedent for this?
No. Keep it here you are getting good responses.

I agree with adding screen stoppers if either of those tubes oscillate you would have problems. It also looked to me like the grid stoppers on the 6V6s might be metal film. I think carbon comp would be a better choice for stoppers.
__________________
If you are not a part of the Solution... you are a part of the Precipitate
  Reply With Quote

Reply


DIY Tube Preamp causing Power Amp Protect ModeHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Cx 1200 D stuck in power protect mode. Austin texans Class D 3 16th October 2013 02:52 PM
fosgate power 1000.1BD turns on in protect mode 63bird Car Audio 12 10th September 2013 09:27 PM
Power Acoustik lfa4-840, no RCAs protect mode RJLatherow Car Audio 14 2nd August 2012 12:37 AM
Audison Millenium Power 4 protect mode benwake Car Audio 4 24th January 2012 02:50 PM
Power acoustik lt1920-2 amp stuck in protect mode psu ishue ivkata Car Audio 15 14th December 2010 02:29 AM


New To Site? Need Help?

All times are GMT. The time now is 12:53 PM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 14.29%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki