When are higher quality parts really worth the money in guitar amps? - Page 1
Forum section: electronics / projects

author: Planobilly
author_rank: Regular Contributor
timestamp: May 11, 2016, 05:11:25 pm
content: Hi,I am starting to design and build a 25 to 30 watt tube guitar amp. To some extent it does not really matter what it cost.Parts like transformers will be for sure high quality and tube sockets will be the best I can buy for example.As most amps are designed to a budget that people will actually buy, there is always cost considerations that come into play. I have no such considerations for the most part. If the amp cost $1000 or $2000, it does not matter to me.How a amp sounds is a function of design and to some extent the quality of the parts. Very subjective subject at best.How reliable the amps will be is a function of design and part quality. Not so subjective.So...what should I over build and where is it most critical to use very high quality parts to get the best reliability?Cheers,Billy

author: karoru
author_rank: Regular Contributor
timestamp: May 11, 2016, 05:46:35 pm
content: Pots, switches and so on. They get abused and nothing's worse than scratchy potentiometer on the stage;)

author: BobsURuncle
author_rank: Regular Contributor
timestamp: May 11, 2016, 06:02:52 pm
content: And capacitors, at least the electrolytics. Use metal film resistors, not carbon comp, especially in the early gain stages to reduce noise.

author: CJay
author_rank: Super Contributor
timestamp: May 11, 2016, 07:30:03 pm
content: Quote from: BobsURuncle on May 11, 2016, 06:02:52 pmIs that such an issue? High fidelity is not generally the aim for a guitar amp, the 'valve' sound is pretty awful from a specification point of view, a little bit of noise from a carbon comp resistor might even be regarded as desirable

author: akis
author_rank: Frequent Contributor
timestamp: May 11, 2016, 07:39:30 pm
content: The "crunch" stage of a valve guitar amp has a voltage gain of 50 (before ANY pots) and the "metal" stage has a gain of 50 x 50. Say the Mesa Boogie amps. The input resistance is 1MOhm as required from passive guitar pickups.At those amplification levels and input resistance you WILL pick up extra terrestrial messages from deep space and you want the MOST quiet stage topology possible, and the best quality materials. Ferrite beads on the input, best quality resistors and caps, alum electrolytics away from heat sources (say the valves!), best quality pots as large as possible, and so on.

author: akis
author_rank: Frequent Contributor
timestamp: May 11, 2016, 07:44:33 pm
content: Quote from: Planobilly on May 11, 2016, 05:11:25 pmI have studied the Mesa Boogie amps, tested a few, bought one (the Rectoverb 25) and have designed a solid state guitar pre-amp that matches the Mesa (if I may say so)One thing they do very well is they add a lot of tone controls, with switches no less, and every guitarist tweaks until he finds his preferred sound. And Mesas are pro stuff, really smooth, really quiet. Unlike, say the Marshalls which sound as if someone is at a brick wall with an angle grinder...

author: chris_leyson
author_rank: Super Contributor
timestamp: May 11, 2016, 08:16:06 pm
content: I think Peavey drive their valve amps so the transformers just start to saturate, there is an article somewhere on the Peavey website. Nice

author: Cerebus
author_rank: Super Contributor
timestamp: May 11, 2016, 10:54:31 pm
content: Quote from: akis on May 11, 2016, 07:44:33 pmAnd what, may I ask, is wrong with the classic sound of British Heavy Metal?Many, many, years ago I used to share a flat with a, then obscure, now well known, British Heavy Metal guitarist - this is why I regard hearing protectors as essential domestic equipment.

author: BobsURuncle
author_rank: Regular Contributor
timestamp: May 12, 2016, 12:04:17 am
content: Quote from: CJay on May 11, 2016, 07:30:03 pmFidelity is not the aim of a guitar amp - as you said, not anymore than a trumpet is designed to linearly amplify buzzing lips. An electric guitar amp is an instrument. Certain kinds and amounts of harmonic distortion are desirable, but hiss is not something one would want to design in. You already get too much noise from single coil passive pickups and various stomp boxes and other sources before your signal even gets into the amp.

author: Planobilly
author_rank: Regular Contributor
timestamp: May 12, 2016, 01:46:51 am
content: Before I started repairing guitar amps I had little idea what was well made and what was not well made. I plugged my guitar into a amp at the store and liked the sound or not. I guess most of us depend on what we hear from others to make buying decisions.My first real amp was a new 1965 Fender Twin Reverb, a blackface. It was pretty well made and lasted many years. I assume it is most likely still around somewhere. I hope someone a lot younger that I am owns it now because you need to be young to carry that heavy SOB around...lol This amp I have in mind to build will stay at home so weight is not an issue.This next amp is something I want to be well though out. Something that will be well designed and well made and look good. Some things I have in mind to do may seem a bit over the top. No Tolex, real leather. My brother in law has a factory that makes the fancy leather used by fashion designers. I have in mind to twist his arm for a bit of the good stuff...lolMy current investigations are the power supply issues. What are really the best design ideas for vacuum tube amps when it comes to power supply. I for sure do not want to use tube diodes. I am not much impressed with sag and that can be had with silicon diodes anyway. I sort of assume I will DC power the heaters, perhaps with a regulated supply. Just gathering information at this stage.Cheers,Billy

author: BobsURuncle
author_rank: Regular Contributor
timestamp: May 12, 2016, 04:03:11 am
content: Quote from: Planobilly on May 12, 2016, 01:46:51 amI share your sensibilities. I too have ambitions to design a custom amp that sounds good and looks good. Not a fan of Tolex, it's very practical for gigging, but if the amp isn't getting hauled around then there are more attractive options since you don't have to worry about it getting dinged up. If you are gigging leather would be very attractive option indeed. Good Luck with your project!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2016, 07:56:32 am
content: From what I've seen about guitar amp designs, the sought-after "sound" comes from the nuances of amplifier bias:- Bias point (how well centered between saturation and cutoff)- Grid rectification (or analogous results for AC coupled BJT amps, if such exist), bias shift (into class C), recovery- Supply sag, and how it affects other stages (usually the low level stages are supplied from RCRC filters)These are specific to tube amp designs. Or shitty 60s era BJT designs, but I don't know that anyone used or uses those in (desirable) guitar amps.Topological and frequency response/distortion considerations:- Negative feedback, whether around a single local stage, or global- Number of stages, total gain; clipping or compression amount- Tone control, and whether it comes before or after (or inbetween!) the high-gain clipping stages- Choice of SE/PP, and types of stages used (e.g., cathodyne vs. paraphase vs. LTP phase splitters)I suspect many of these attributes could be very accurately, and moderately easily, reproduced with fairly simple BJT designs. Or extremely complex DSP designs, plus who knows what other fancy effects (who needs pedals?). There's no technical reason for tubes anymore, though it's still hard to beat that feeling of jamming around a warm campfire that only tubes can provide.Tim

author: akis
author_rank: Frequent Contributor
timestamp: May 12, 2016, 11:03:10 am
content: Quote from: T3sl4co1l on May 12, 2016, 07:56:32 amI think the most important aspect is the quality of the distortion afforded by the valve pre-amp stages and then a little bit the extra distortion at the output valve stages where you have to turn it really high to make an impression (to those output valves). Then there is the distortion from the insufficient power supplies and output transformers and even the torn speakers. You must blast it to hear a difference.But you can get the most beautiful crunch or metal sound from the pre-amp and the only element active there is simply the few pre-amp valves - all else is inconsequential.And there is no magic elements, simply the way an Av=50 pre-amp valve stage clips very softly and compresses smoothly with huge headroom of 100s of volts. You can approximate all that with an array of dozens of Schottky diodes but the ridiculous headroom is harder to emulate.

author: CJay
author_rank: Super Contributor
timestamp: May 12, 2016, 12:17:14 pm
content: Quote from: BobsURuncle on May 12, 2016, 12:04:17 amI'm not suggesting one would design in hiss, rather I'm advocating not being overly bothered by the amount of hiss you could design out, most electric guitar players I've met would kill for a classic Marshall or other such valve amp, complete with carbon resistors, some of which sounded like a radio tuned to dead air.Maybe I'm wrong but I can't remember ever meeting a guitarist who complained about hiss.Keyboards and vocals, that's another matter entirely of course.

author: John Heath
author_rank: Frequent Contributor
timestamp: May 12, 2016, 12:21:29 pm
content: A 12au7 or higher gain 12ax7 tube is popular in guitar amps. The 12 in 12au7 means filament voltage 12 . It occurred to myself that you could take advantage of this as op-amps also require 12 volt rails. The 12 volts is already there for free. Use the op-amp 12 volt rail to light the tube filaments. Place a 12au7 tube in a place of importance , maybe a gold frame with glass window so glowing filaments can be admired by all. This will make it clear that we mean business with this guitar amp , money is no object. Now that the horse and pony show is done we are liberated to design a precise amp with op-amps and mosfet outputs. Technically this is a tube amp as it does have a tube.

author: akis
author_rank: Frequent Contributor
timestamp: May 12, 2016, 12:41:32 pm
content: Quote from: John Heath on May 12, 2016, 12:21:29 pm2 * 12AX7 in series with Av=50 each, powered by 300V-400V rails. The dynamic range of the guitar's pickups is enormous and then amplified by these multipliers. I do not think that the glowing tube(s) are just for show. This does not mean that valve amps all sound great, eg look at "Marshall" for the most faithful reproduction of the sounds made by angle grinders and other industrial tools!Also I doubt that the 12V op-amp supply would be able to provide 100s of mA to heat those valves anyway. :-)

author: Cerebus
author_rank: Super Contributor
timestamp: May 12, 2016, 01:53:35 pm
content: Quote from: CJay on May 12, 2016, 12:17:14 pmThat's because after a few years of gigging guitarists have a hearing threshold of 60 dB SPL. You will find recording engineers who have to record the things complaining about it though. It's quite common practice to sling the amp in another room and mic it up so that it can be driven hard enough to get the desired 'sound'. Put a few of those together and it can really ruin the noise floor obtainable on a recording.

author: BobsURuncle
author_rank: Regular Contributor
timestamp: May 12, 2016, 05:30:47 pm
content: Quote from: CJayFunny you should mention, at the bar last week my buddy was complaining about the hiss from his Crate amp and I was debating whether to buy the new Fender Elite Strat with next gen noiseless pickups and or to swap them out for Seymour Duncan SSL-1 and -5s pickups which are noisy but have a little better tone.Noise is an unwelcome stowaway that comes with a number of designs that are otherwise highly prized, whether it be an amp, pedal or pickup. If metal film's were available back in the 50s and 60s at today's prices those old vintage amps would never have used carbon comps.

author: Shock
author_rank: Super Contributor
timestamp: May 12, 2016, 09:09:07 pm
content: Quote from: Planobilly on May 11, 2016, 05:11:25 pmCheck out Hiwatt amps these are from the 70s and the reliability is second to none.