Go Back   Home > Forums > Amplifiers > Tubes / Valves

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

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 9th December 2008, 12:02 AM   #1
diyAudio Member
 
Join Date: Jun 2008
Default Improving low bass of a Scott 299c

I recently got a Scott 299c serviced and functioning, and over all I'm very happy with it, but it is definitely a bit on the bright side for my tastes, and the low end seems to be rolled off quite a bit. It's there, but it doesn't have much body. This is more pronounced on the phono input than the line in, but it seems a bit rolled off on the line input from what I've heard so far (I'm mainly a vinyl listener these days, so I haven't played many CDs through it).

The preamp tubes are the original telefunkens, but the rest of the tubes have been replaced with new JJ equivalents (the rectifier was shattered when I got it, and the rest were really weak). According to my tech, the bias rectifier was already silicon rather than the expected selenium. I believe he replaced the ceramicap oil caps, but other than that it's pretty much stock.


Any ideas for improving the low end with minimal modification? I know there's an always-on highpass in most Scott amps, which seems like a prime candidate.

EDIT: The simplest option might be to build a small subwoofer and run it off of the powered center channel output of the Scott, but I would be more inclined to do a simple modification, for cost and space reasons.
  Reply With Quote
Old 9th December 2008, 01:30 AM   #2
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Here's a useful site and a link to the schematic for the 299C: http://hhscott.com/pdf/299C.pdf

I've worked on quite a few of these and never heard of a non switchable high pass filter in a Scott amplifier, although I will concede that the corner frequencies of the various interstage capacitors were chosen to limit infrasonic response to prevent output transformer saturation. It would be unwise I think to deviate significantly from the original design intent for this reason.

This model does have a switchable rumble filter.

I would suspect that most if not all of the electrolytics in the amplifier need to be replaced if this has not been done already. This includes cathode bypass caps in various low level stages as well as the power supply capacitors.

The bias and dc filament supply capacitor (can type) is unconventional in that it utilizes a common positive and cannot be replaced with a standard common negative type. The others are conventional.

Bass performance with appropriate speakers is quite good, but not in the head banging class, and in real terms this amplifier is capable of no more than 30Wrms per channel and probably less if the power supply is tired. This means you need an efficient speaker system and one that does not mind an amplifier with a low damping factor - otherwise the bass can be more than a bit tubby.

There are system integration issues to contend with here, and you have not mentioned what other components you are using this with, nor the speakers used.

Something else you might not know is that the tape out on this and most Scott amplifiers is unbuffered which makes the amplifier incompatible with almost all modern recorders with line input impedances of less than 100K or so.

Make sure that the amplifier is really up to snuff first, and then if necessary add that subwoofer if you need head banging bass - otherwise you will need something compatible from the era that was noted for high efficiency and good bass, something like a speaker system using JBL D130 bass drivers for example.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 9th December 2008, 04:10 AM   #3
diyAudio Member
 
Join Date: Jun 2008
Hey,

I know some caps were replaced, but I didn't get to talk with my repair guy as long as usual because he was auditioning an amp for someone else when I picked mine up, so I don't actually recall the specifics. The can caps were left stock, and he tested the rest of them and replaced anythig that was severely off value or leaky.

I'm using it with a pair of AR 4a's, which don't put out a ton of bass but definitely put out more with my old Technics SA700, and were if anything a little on the dark side (with this, I'm actually running them with the tweeters rolled off a bit). For a source, my turntable is a Technics SP-15 with the stock EPA501 tone arm and a Grado Prestige Red. For CDs I'm using a Playstation 1 (which really lived up to the hype), which seems over all pretty well balanced.
  Reply With Quote
Old 9th December 2008, 01:18 PM   #4
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
I'm not a real big fan of the AR-4A, and I would recommend you focus your attention on upgrading those relatively soon. They just seem to perform better with solid state amplifiers.

The original supply electrolytics in your 299C at this point should be quite suspect whether or not they actually function "ok" - they're now about 46yrs old and well beyond their design lifetime. ESR has undoubtedly increased greatly, capacitance decreased significantly and this would contribute somewhat to the lackluster bass performance you mentioned.

Yeah, amazing that the PS1 is as good as it is, the PS2 is not in the same league sonically.

You might also try looking for a NOS or good used set of US made 7591..

Send me a pm sometime, I see we are geographically close..
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 9th December 2008, 03:41 PM   #5
diyAudio Member
 
Join Date: Jun 2008
Hey, thanks!

I'm pretty strapped for cash right now, so no major upgrades or repairs in the cards for the near future.

I've got a set of AR2a's that need to have the rheostats replaced and the crossovers recapped, which I ca do myself no problem but I've been piutting it off becasue it's such a pain to get those things open. What's your experience with them like? I know the 4a's are pretty inefficient. I listen to a lot of 60s and 70s vinyl, so I'm pretty in to the idea of using vintage speakers of some kind if I can get a decent sound out of them, but I do plan on building a simple set of efficient full-range speakers (probably the "metronome" frugalhorn design, since I don't have the tools to do anything more complex right now) so that down the road when I ahve the cash and time to try my hand at building a SE tube amp I'll have someting i can actually drive. Maybe I'd be best off making he speaker project a higher priority. I'm going to give a set of salad bowl spheres inspired by some of the threads on here a shot as a gift for my parents this Christmas, so we'll see how that goes.

For what it's worth, now that I've got jsut a couple more hours of play on the new stylus and tubes, the bass does seem to be opening up a bit, but I still ahve to keep the bass controls at +2 to get anything like a pleasant balance.

Edit:

I'm doing some more CD listening to refresh my opinion of the situation, and there's actually not trouble at all with CD, so this is either an issue of the phono preamp needing work, some kind of mismatch between the cartridge and preamp, or possibly a setup problem on the turntable (although I'm pretty confident that the turntable's set up pretty well, I put a lot of time in to tweaking it last week, and it's pretty dead on at this point, at least to the degree of accuracy I can get it with the tools I have). Someone in another thread mentioned a capacitance mismatch being one possible source of a poor balance between highs and lows, but I have read a few things that suggest adjusting the capacitance of the phono input only works with moving coil cartridges.
  Reply With Quote
Old 9th December 2008, 03:59 PM   #6
diyAudio Member
 
Join Date: Jun 2008
My first instinct would be to lower the arm a bit, but it starts to get muddy and two dimensional if I lower it much. Right now, it's just a little lower than the height that would make the arm parallel (and the front of the cartridge perpendicular) to the vinyl with a record of average thickness, and listening seems to confirm that this is the best sounding height for it with most albums.

Lowering it a LOT (from just over 19mm wher it is now all the way down to 14mm) seems to give a better balance over all, but it is very lifeless, lacks dynamics, and is a little dull in the highs, so that doesn't seem to be the answer. Pretty much everything in between sounds either brittly and shrill or muddy in the mids.

I'm living in Providence RI right now, incidentally.

I'm going to do some more listening to some newer stuff, actually.

It's very possible that what I'm hearing as a lack of bass is actually a combination of the typically light bass in older rock recordings and the increased high end of this amp compared to my old one.
  Reply With Quote
Old 9th December 2008, 04:33 PM   #7
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
Quote:
Originally posted by ALustyGhost
My first instinct would be to lower the arm a bit, but it starts to get muddy and two dimensional if I lower it much. Right now, it's just a little lower than the height that would make the arm parallel (and the front of the cartridge perpendicular) to the vinyl with a record of average thickness, and listening seems to confirm that this is the best sounding height for it with most albums.

Lowering it a LOT (from just over 19mm wher it is now all the way down to 14mm) seems to give a better balance over all, but it is very lifeless, lacks dynamics, and is a little dull in the highs, so that doesn't seem to be the answer. Pretty much everything in between sounds either brittly and shrill or muddy in the mids.

I'm living in Providence RI right now, incidentally.

I'm going to do some more listening to some newer stuff, actually.

It's very possible that what I'm hearing as a lack of bass is actually a combination of the typically light bass in older rock recordings and the increased high end of this amp compared to my old one.

Could be all of those things.. I used a Grado Reference Platinum and have found it quite finicky in just the same areas you mention. I have also noted that it takes a while for a new Grado to break in and sound its best. I've got about 50hrs on mine and it is still improving - at one point I thought it was pretty uninvolving sounding and the SME3009 arm I have is difficult to set up and less tweakable than you might suspect..

I would think the 2A might be a better choice for this amplifier, but it is one model I have not heard much of. I have found the Clarity SA film caps to be a reasonable choice in X-O recaps and used them when I designed and built my 2nd gen Onken X-O late last year. You might get away with just cleaning the controls, not always the case however.

Get Clarity caps here: http://www.madisound.com/catalog/ind...=404_5_351_352

I'm all diy, and currently use 300B SE amplifiers based on one of my old commercial designs with my homebrew Onken set up.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 9th December 2008, 05:04 PM   #8
diyAudio Member
 
Join Date: Jun 2008
Maybe I should swap in my old stylus. It probably has 300 hours on it, and I only replaced it because I was getting all kinds of distotion in the upper midrange and only later discovered that a previous owner had actually hooked the green and blue ground leads on my tone arm to the wrong channels, so the left and right grounds were swapped. Didn't do anything (or was so subtle I wrote it off as being the fault of the amp) with my SA700, but raised hell with the Scott until I noticed it. Anway, my old stylus probably has plenty of life left in it (I found I was able to get about 12 months of fairly heavy use from the previous two, but that one's only 5 months old).

I'm definitely noticing that newer stuff that's mixed with less emphasis on the upper mids seems OK with only a slight bass boost on the tone conrols (which I prefer to run flat, but whatever makes it sound best). I might just need to adjust to the better high end this amp has, maybe listen from farther back and bring the volume up a bit, too, since there seems to be a sweet spot where the amp really fills out, but it's just a bit too lout for comfortable listening with bright mixes. Turning on the "loud" switch actually gives it a really nice balance across the spectrum, but pretty much kills the sound staging and dynamics, so that's out.

Play wear on some of the rarer albums I've got that I wasn't able to find clean copies of is a lot more obvious with this setup, too.

Probably, I should listen more before I start speculating, is the lesson here!
  Reply With Quote
Old 9th December 2008, 05:25 PM   #9
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
One thought would be to slightly turn down the treble control to tame that brightness you mention. This is one of the fun aspects of a vintage integrated amplifier - just use a very small amount of attenuation.. Also the 4AX had a tweeter level control (pot or switch) and I am assuming the 4A does as well - I would try this as well.

Make certain that your speakers are phased correctly and that the speaker phase switch is not in the "Reverse" position unless one of your speakers is connected out of phase..

You can try the older stylus and see if it makes any difference - and it might.. Something else to think about is the tracking force you are employing. Always set it to the top of the recommended tracking force range - this generally enhances tracking and this is particularly noticeable - you guessed it - in the treble. I know you have spent a lot of time on the geometry issues so perhaps just a quick recheck is in order.

At the risk of opening a can of worms you might want to take a look at what you are using as speaker cables. Flame suit on! IMHO zip cord just does not cut it, but there are a lot of other options starting with just simple twisted pairs you can make yourself. I favor litz wire which you can sometimes find surplus - if nothing else use at least 16ga wire (zip) of equal length, and no longer than that required to reach the furthest speaker. Make sure your speakers are connected in phase.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 9th December 2008, 05:32 PM   #10
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
I have some old albums that I have played so much that even with the generally good turntables and arms I've had over the last 35yrs or so I get some distortion and not always in the most obvious places either.

Unfortunately for me my system is absolutely and gruesomely unforgiving - flaws mercilessly revealed, that is of course in addition to the system specific flaws I am already aware of, but OTOH I guess I would not have it any other way..

This is a great hobby, the only one in fact that has kept me continuously entertained since my teens.. I love music, and like making things, what better combination could there be?
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Improving bass extension: Mission Argonaut Mk 2 matski Multi-Way 5 4th May 2011 02:13 PM
H.H Scott S 71 Xib Multi-Way 1 22nd March 2007 05:17 PM
improving bass response - yamaha speakers s115ivs sidpro Multi-Way 1 3rd April 2006 07:40 AM
Scott's FE167 ML-TLs SCD Full Range 9 24th February 2004 07:31 PM
Scott amp ???? rwagter Solid State 0 8th September 2003 04:07 PM


New To Site? Need Help?

All times are GMT. The time now is 02:15 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2