Tube amp questions - Page 1
Forum section: electronics / projects

author: sahko123
author_rank: Frequent Contributor
timestamp: November 03, 2018, 05:13:51 pm
content: ok then,ive started designing a tube amp and I need to know this. is it a good idea to drive higher impedance loads say 64 ohms for a set of headphones with an output impedance of 8 or 16 ohms from an output transformer? ive googled it and I cant find a concise answer would there be big discrepancies in volume/voltage or problems with efficiency?

author: iMo
author_rank: Super Contributor
timestamp: November 03, 2018, 05:30:57 pm
content: https://www.diyaudio.com/forums/tubes-valves/264623-output-transformer-impedance-matching.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 03, 2018, 08:27:47 pm
content: Impedance matching doesn't mean much when you need mW from a multiple-W amplifier.More important will be output voltage stability and frequency response, which often depends on load impedance.Tim

author: spec
author_rank: Frequent Contributor
timestamp: November 03, 2018, 09:44:36 pm
content: Quote from: sahko123 on November 03, 2018, 05:13:51 pmYou would probably get away with it, but there is a danger of damaging the output transformer insulation. Best not to take a chance and fit a resistor, equal to the specified output impedance, across one of the transformer outputs. Then connect your phones across any of the outputs (16 Ohm output will be louder). Take it easy on the volume and check if the phones are excessively loud. If so, you would need to fit an attenuator for the phones to protect the phones and your ears.A 20W or higher power resistor should have enough power handling capacity for amps up to 50W.There are a few valve addicts on EEV so don't be shy about posting details of your amp build

author: sahko123
author_rank: Frequent Contributor
timestamp: November 04, 2018, 03:57:34 pm
content: Thanks for the multiple responses I was thinking would be able to put a resistive load in parallel or series with the headphones so the transformer sees the desired resistance or does the impedance of the headphones vary enough through the frequency ranges that this would be a bad idea? I want to know as much as possible before putting any thing together and blowing something. for those who want to know the headphones are 55ohm Akg K240's and the tubes are 6550s with preamp tubes to be decided.

author: spec
author_rank: Frequent Contributor
timestamp: November 04, 2018, 09:38:07 pm
content: Quote from: sahko123 on November 04, 2018, 03:57:34 pmParallel is the way to go. That is what I implied in the previous post.The load seen by the output transformer does not have to be that exact.Quote from: sahko123 on November 04, 2018, 03:57:34 pmApart from blowing the output transformer by running the amp into an open circuit, valve amps are pretty rugged. One thing to take special care with is the output valve biasing. If you loose the bias voltages the output valves will over heat (red plate as they say) and get damaged,Nice valves: sounds like you will be spending some cash

author: sahko123
author_rank: Frequent Contributor
timestamp: November 04, 2018, 10:20:52 pm
content: 6550s are pretty over kill but I got 12 of them for an excellent price so i'm not complaining

author: Circlotron
author_rank: Super Contributor
timestamp: November 04, 2018, 10:39:42 pm
content: 6550s for a headphone amp?Nothing exceeds like excess.

author: spec
author_rank: Frequent Contributor
timestamp: November 05, 2018, 05:32:34 am
content: Quote from: sahko123 on November 04, 2018, 10:20:52 pmQuote from: Circlotron on November 04, 2018, 10:39:42 pmThis is a headphone amplifierI didn't realise that. I thought it was a guitar amp with the facility for plugging headphones in.A headphone amp is a quite different proposition.Are you aiming for HighFi or are you after one of the valve sounds?

author: sahko123
author_rank: Frequent Contributor
timestamp: November 05, 2018, 04:43:11 pm
content: id like for high fi but theres a reason its not solid state

author: sahko123
author_rank: Frequent Contributor
timestamp: November 05, 2018, 04:49:03 pm
content: Now I have another question not to do with impedance. This question is to do with powering the tubes. I was wondering of I could use rectified mains for the high voltage line and use a smaller transformer just for the filament voltage.(I live in Ireland with a 220v mains)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 05, 2018, 04:52:29 pm
content: Not directly, that would be stupid.You can, however, use two transformers, back-to-back, to get filament and high voltage, both isolated, without having to find a specialty part.That said, Hammond (and a couple of smaller manufacturers I think) make standard transformers for just this sort of thing, that aren't much more expensive than a standard (single output) transformer, and are carried by most distributors. YMMV.Tim

author: sahko123
author_rank: Frequent Contributor
timestamp: November 05, 2018, 05:02:09 pm
content: k I found well priced transformers that push both filament and plate voltage I thought that would be possible to go for a cheaper option

author: spec
author_rank: Frequent Contributor
timestamp: November 05, 2018, 05:21:02 pm
content: Quote from: sahko123 on November 05, 2018, 04:49:03 pmYour best bet is to use an off-line switch-mode power supply to get a nice stabilized supply line of anywhere between 200V and 400V. These off line switch mode PSUs are compact, will work off any voltage from 90V to 270V and are reasonably cheap too. You can use another mains PSU to provide 6.3V or 12.6 DC for the heaters.As you are building a headphone amp, you can consider going full class A single ended.You could also consider doing away with the output transformer.If you are interested in the off-line switch-mode PSU approach, I can post a schematic of one of the designs I did a few months ago, which may give you a few ideas.

author: schmitt trigger
author_rank: Super Contributor
timestamp: November 05, 2018, 05:32:42 pm
content: Quote from: Circlotron on November 04, 2018, 10:39:42 pmHe could have used KT88sNow seriously......in the mid 1960s vacuum tube amplifiers seldom came with headphone amplifiers. A popular hobby shop at the time, Calectro Electronics, would sell a headphone adapter box. It was essentially a DPDT switch, to select either the speaker or the headphones. In headphone mode it had -you guessed it- a pair of 8 ohm power resistors in parallel with the headphone jack.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 05, 2018, 06:06:42 pm
content: Quote from: spec on November 05, 2018, 05:21:02 pmI'm a bit interested (maybe put it in another thread) -- does that come with winding details of the transformer, and EMC test results? Theory says (and backed up by my own measurements) that high voltage outputs are that much harder to pull off cleanly than low voltage (where you only have the primary's voltage and ringing to deal with).Tim

author: sahko123
author_rank: Frequent Contributor
timestamp: November 05, 2018, 06:11:32 pm
content: I would have used kt88s but that would have involved buying them. the 6550s where used for 5 euro 12 of them so I thought why not and they were shown to me to be all working so I couldnt let an opportunity like this pass. (Plus they look cool as hell)as for the Offline switch mode psu im not even sure where to start im not sure what they really are. I understand what switch mode supplies are but are hese ones specifically for powering vacuum tube circuits or other high voltage dc electronics?

author: chris_leyson
author_rank: Super Contributor
timestamp: November 05, 2018, 08:26:43 pm
content: RS Components sell a good range of OEP audio transformers everything from a few watts up to 25W, the larger transformers also have 20% taps for ultra linear operation. I breadboarded a single ended 3W amp using an OEP N35A002F and a 6BW6 with a 25% ultra linear tap, I don't know what's happened to it and I didn't get around to making any distortion measurements. Never mind, I've found another N35A002F so I will probably have another go at it, single ended or push pull. In push pull I can drive twice as many volts into the windings then I can push the core close to saturation. It's something that Peavey did with some of their amps, not only do you have valve sound but you got B/H loop sound as well.

author: Gyro
author_rank: Super Contributor
timestamp: November 05, 2018, 09:25:12 pm
content: Quote from: sahko123 on November 05, 2018, 06:11:32 pmThese days the economical alternatives are Soviet NOS GU50s, they look even cooler once relieved of their hot-swap (really!) knobs and lids. The only downside is 12.6V heaters (but half the current). Built like (for) tanks with hard glass envelopes and 4 getters!Strangely, once de-lidded, they look much more modern than the more recently designed KT88. - see attached (before and after).P.S. It's wise to cook them for a while to mop up stray gasses from cold storage before applying HT.

author: sahko123
author_rank: Frequent Contributor
timestamp: November 05, 2018, 09:47:56 pm
content: they do look really cool but I enjoy the smooth tops of the 6550s and nos tubes wont be around forever

author: Gyro
author_rank: Super Contributor
timestamp: November 05, 2018, 09:51:27 pm
content: I know what you mean. Supply isn't that much of an issue though - China bought up the tooling and still makes them as the FU50. I suspect that their military might still use them, probably not quite of the same quality though. EDIT: Germany started the whole ball rolling with the Telefunken LS50 during WWII, then the Russians captured some...https://en.wikipedia.org/wiki/GU-50

author: sahko123
author_rank: Frequent Contributor
timestamp: November 05, 2018, 10:02:57 pm
content: thanks for bringing these up but ive not much money to spend on this(High school student with fuck-all else to do with his spare time besides EE projects) its mostly going to the power and audio transformers along with some passives like resistors capacitors depending on what I do and don't have. I've wanted to do a project like this a while but didn't want to spend too much on good tubes so I was waiting for cheaper used tubes. Then the opportunity arrived and I bought them they are 6 sovtek 6550s and 6 jj 6550s. The same seller has preamp tubes that im also thinking of buying.(12ax7's) This all depends on the design I go for. Im designing the amp more so around these tubes with necessary additions or other thinks I might find that I would like to include. But if you have anything to suggest to a tube virgin then feel free. ive been doing solid state stuff for years and have wanted a bigger challenge.

author: Gyro
author_rank: Super Contributor
timestamp: November 05, 2018, 10:11:57 pm
content: QuoteIf short of money, then also check out 6n1p-ev and 6n2p-ev (preferably Voskhod), again rugged mil spec and superior but slightly different heater connections to the 12ax7 - 6.3V heater connection only, with pin for internal shield.EDIT: ... but from the prices you're quoting, it sounds as if you'll get a decent price anyway.

author: spec
author_rank: Frequent Contributor
timestamp: November 05, 2018, 11:30:04 pm
content: Quote from: T3sl4co1l on November 05, 2018, 06:06:42 pmNot sure why you ask about winding details, but no, they are not provided.Never had any problem with noise. You can get as smooth as you like by adding local CR filtering any way.Don't forget that OLSMPSUs are everywhere. My Teck scope even has an OLSMPSU, and every light bulb in the house has an OLSMPSU- I know because when the glass bulb fails I salvage the components.

author: spec
author_rank: Frequent Contributor
timestamp: November 06, 2018, 12:55:59 am
content: Quote from: sahko123 on November 05, 2018, 06:11:32 pmNo, OLSMPSUs are not specifically for valve amps.I will see if I can post some information about OLSMPSUs.

author: Zero999
author_rank: Super Contributor
timestamp: November 06, 2018, 09:32:55 am
content: Quote from: sahko123 on November 05, 2018, 04:49:03 pmYes, that's possible in theory, but it would be very dangerous. Headphones and speakers don't have adequate insulation for mains voltages, so whilst it will work in theory, in practise it's highly likely to cause someone to be electrocuted. If everything connected to the amplifier (both inputs and outpus i.e. microphones, speakers, headphones etc.) via transformers with sufficient insulation between the primary and secondary windings, when such a set up could be safe, but in practise common audio transformers don't have mains rated insulation.

author: sahko123
author_rank: Frequent Contributor
timestamp: November 06, 2018, 04:56:47 pm
content: If I do go for an olsmpsu it would be in off the shelf module form because this is a one off thing and I would likely be doing this on tag boards rather then fleshed out pcbs in which I could add the circuitry required

author: spec
author_rank: Frequent Contributor
timestamp: November 06, 2018, 08:39:33 pm
content: Quote from: sahko123 on November 06, 2018, 04:56:47 pmEssentially yes, the modules are built and off the shelf. I would like to call this approach switching power supplies from now on. OLSMPSU is cumbersome and a tad misleading.When you see my post, currently work in progress, describing the switching PSU approach, all will be clear... I hope.

author: spec
author_rank: Frequent Contributor
timestamp: November 06, 2018, 08:46:14 pm
content: Quote from: Hero999 on November 06, 2018, 09:32:55 amWise words H999

author: spec
author_rank: Frequent Contributor
timestamp: November 08, 2018, 05:22:46 pm
content: Attached is a Word.doc summarizing the proposed switching PSU. I hope it explains the approach clearly and answers any questions you may have.